The problem with pretty much any filament is that you'll end up with something that either leaks or slowly seeps - especially at any sort of pressure. You can maybe coat the inside with a waterproof epoxy to get it hopefully water-tight. I did see an interesting concept of using a laser cutter to cut acrylic and then a 3D printer to print moulds for 2-part silicone that both waterproofs it and acts as the gasket. It looks a lot of faff but it's an interesting idea:
For the thread milling, I modelled it in Fusion and then got it to generate the G-code. Picture anddetailsrant on my Overkill thread if you're interested. Import carbide thread mill suitable for G1/4 and G3/8 threads for £14 (the d8 variety) and pray! Best guide to the weirdness'es (and why!) of Fusion when it comes to CAM for thread milling is NYCNC's....'cos it's anything but intuitive. It makes sense once you understand the logic but considering you can drop a modelled thread in with Fusion supplying all the dimensions and then hit 3D print, I'm not sure why it isn't as easy for CAM to be honest. Still, it works once you know the ritual.
Love the idea in that video, never though of it like that to create a silicone gasket within the acrylic for the coolant to run through. I do wonder though with that method, what kind of silicone based product you would need to stop mould and other things building up... Though technically if you use the right mixture it should never happen.
Fusion is the way to go, it's pretty much what i use for everything if im modding or creating a custom part for a build. In Fusion you can create holes with the appropriate threading already so you don't have to model it which is fantastic and a nice time saver lol.
Interestingly on the topic of CAM, what file format do you normally use? I've asked a few older milling places around my local area and they seem to want PDF and have no idea what i mean when i show them a STEP file.
) and see if it offers any improved features that are currently lacking - mainly in probing, co-ordinate display and offsets.