If it's any help, I used the below plywood size as I wanted plenty of room. You could also save money and not get an all-in-one cased heater; I just went for the easy option coz I'm lazy 
12mm Marine Plywood
2 x plywood-sheet-12-marine-plywood-l700-w700
2 x plywood-sheet-12-marine-plywood-l700-w350
1 x plywood-sheet-12-marine-plywood-l700-w400
12mm Marine Plywood
2 x plywood-sheet-12-marine-plywood-l700-w700
2 x plywood-sheet-12-marine-plywood-l700-w350
1 x plywood-sheet-12-marine-plywood-l700-w400
Deleted User 298457
Deleted User 298457
OP
That's brilliant insight, thank you! I am always pained to touch my workshop as the previous chap basically had it built and then never so much as put a drawing pin in the structure lol.
It should have minimal effect on your workshop - for mine it's just 4 screws fastening the backboard of the box to the wall, an 80mm hole I used a core drill head for to get the hot air pipe into the garage/workshop and a 6mm drill hole to take power out from a normal 3pin socket to the PSU. I just used a 3m kettle lead, cut the moulded kettle-type plug off and connected it up to the PSU. It makes it easy to power up or isolate from the inside, and the Bluetooth app means I don't have to open the box to change any settings.
I'm not an expert and it was the first time I've done this but shout if you have any questions I can help with.
As an aside you can get the marine plywood (or other options) from these folks. they deliver and will pre cut to size/shape/add holes etc if you want to make building the box enclosure easier.
www.cutmy.co.uk
12mm Marine Plywood Sheets
From £2.00 Cut to size Lacquering available Max 2420mm x 1200mmSustainably sourced
www.cutmy.co.uk
Right side of box with front on showing heating air intake
Would it not be beneficial to pipe the heating intake back into the room? Surely it would make the heater a lot more efficient by reheating the existing warm air instead of heating cold air?
When I first ordered mine I decided this was my plan but after researching it's advised not to.
You will get a condensation build up if you recycle the air.
Fair enough, i thought there must have been a reason not to.
I'll explain it to the best of my ability as it may help others who have a humidity problem.
Cold air holds less moisture than warm air. So sucking in cold air, heating it up and then pushing it into the room creates a pressure. This pressure then pushes air out of any gaps in the room and reduces the humidity. Less humidity makes the room feel warmer at the same temperature.
Recycling the air means there's equal pressure and outside air can pass through the gaps and after a time the humidity will increase.
Given I have mine inside the garage it is heating, I have always recycled the heated air. I monitor and log the humidity in my garage continuously (24/7/365) and have a permanently plumbed dehumidifier automated by the monitoring for if it ever gets out of hand. I can tell you with absolute certainty that running my heater for any length of time - from a few minutes to a few days straight - does not materially increase the amount of moisture present in the air in my garage. A period of sustained rain followed by a warm day has 300-500% more impact on the airborne moisture levels in my garage than running the heater. My being in the garage has more of an effect than running the heater....
It has rained all day here, RH has gone up 6% vs a pretty stable 11.9*C temp. The dehumidifier will bring that back down below 60% when it kicks in. Comparing that to an 11h period during a dry(ish) spell where the heater kept the temperature stable at 11-11.2*C during which period the humidity fluctuated between 57.7% at the start and 56.2% at the end with a peak of 61.4% roughly in the middle. The position that recycling increases humidity isn't supported by the data. Oh, and my setup generates a negative internal pressure for the time being - the worst possible case!!
Saying that recycling the air is guaranteed to lead to increased humidity isn't accurate in my experience. This is probably why we don't duct ice cold air from outside to the bottom of our radiators in winter...
That said, all things being equal having a positive internal pressure is a good thing vs a negative internal pressure so that's a thing in the "pro" column I guess.
It has rained all day here, RH has gone up 6% vs a pretty stable 11.9*C temp. The dehumidifier will bring that back down below 60% when it kicks in. Comparing that to an 11h period during a dry(ish) spell where the heater kept the temperature stable at 11-11.2*C during which period the humidity fluctuated between 57.7% at the start and 56.2% at the end with a peak of 61.4% roughly in the middle. The position that recycling increases humidity isn't supported by the data. Oh, and my setup generates a negative internal pressure for the time being - the worst possible case!!
Saying that recycling the air is guaranteed to lead to increased humidity isn't accurate in my experience. This is probably why we don't duct ice cold air from outside to the bottom of our radiators in winter...
That said, all things being equal having a positive internal pressure is a good thing vs a negative internal pressure so that's a thing in the "pro" column I guess.
Do you know if they are T8 or T5 Fluorescent tubes, and if they are singles or doubles? As you say, even in 2024 these lights can be pretty decent. They can fade over time though. The T5 type are more efficient but the wattage can vary wildly depending what you have. 20 up to 80watts for the higher output ones. Assuming they are each say 54watt single T5 tubes as an example, that would be 750 watts if all powered on. That's about 19 pence per hour. If you spent say 10 hours a week in there with them all on, that's about £8 a month. Obviously if they are 80watt doubles then that number goes up a lot.
I'd recommend LED battens. You will need generally less of them, plus you can get fully integrated ones behind a safe cover. Instant on and instant full brightness without the starter delay.
If you do change them out, just another thing to be careful about with any workshop lights in case you didn't know - since I know you do DIY and I think I saw a chopsaw in one of your posts - is that in certain scenarios in some installations, fast spinning machinery can look stationery. You know like the helicopter blades vs camera shutter speed thing? Stroboscopic effects. So you need to be really careful of that. Just do some tests to make sure. Very small chance it could happen but just thought I would put it out there.
Deleted User 298457
Deleted User 298457
OP
Thanks for doing the maths. I will check - but even those numbers alone fill me with confidence. They are on at most an hour a week at the moment (I don't get in there that much at the moment due to new baby) and there are at least 9, maybe 11.
Haha that's nuts - great info. Hadn't considered that but it makes total sense.