Alright. I've just checked and it was very early January when the acetal turned up...so I've literally been waiting all year to cut it. Why? To get it square, I need to set up a table that's got a nice precise grid of holes...and that doesn't fit indoors. Between house-hunting and the fact that it's rained damn near every time I've even looked outside, this has been the first opportunity to do....two cuts. This is why I want some indoor space. With stuff set up, this would have been less than ten minutes work....not the better part of half a day. While I was set up, I also rounded off the corners. I could have done that inside but I might as well make just one mess to clear up rather than two. The corners were routed with a 3D printed guide.
Why that radius, haven't I missed the corner of the actual bed? Yes...but for once, it's deliberate!

With the oversized (for the size of spindle) spoilboard cutter, that's as far out as I can reach with the spindle. Otherwise, I'd end up with towers in the corner as I started to resurface the board.
I drilled and counterbored the holes to mount it using another 3D printed jig. I outdid myself though as I managed, somehow, to get one of them in the wrong place and all of them the wrong size *facepalm* Honestly, I'd modelled M6 threads even but when I checked, an M5 screw threaded in nicely...or appeared to. I even had a bag of M5 screws ordered ready. So I had to do another jig and plunge an endmill to 'move' the hole to the right place - a drill would largely have followed the existing hole.
The rest needs to be done by the CNC itself....but I suspect that's going to have to wait until I move house*. I just don't have the floor space (let alone desk space!) to run it at the moment. Or, to be strictly accurate, I do...but not in a room that I want to fill with plastic chips for a 3m radius!

Doing that in an office I share with my wife....might not go down so well! Can't afford a divorce as well as a house move!
*Edit: I should mention that the top of the chain fell through so we're waiting for that to be completed again before we all start moving again.