Hi
Nothing so complicated! See our build album
https://photos.app.goo.gl/QPiv6sVDTUp1vuTMA - currently about 2/3 down you'll see me playing with the plasitc
Its all just "bent" with templates, straight edges, clamps, and hot air gun. The corners are folded in the same way you'd wrap an xmas present, so folds with an excess V - and this is just clamped up and squishes in. So totally waterproof even before adding PU gunk in the corners, behind and between the walls, and another layer before the sealant.
The tray is in 100% contact with a 15mm play base. So the plug hole, I just made a hole, then chamfered the edges, then raised the ply 10mm keeping the "hole cut out" in the plug hole (now 10mm lower) Then hot air gun, and just pushed the hole downwards (with another "hole cut out" on top to push it down. Once finished, hole saw.
It was my first attempt and "fairly" easy. Its not perfect, do not profess otherwise, but some things went well, some not, but didn't think v2 or v3 would actually improve overall.
It went together in the room really well - so the end product is better than the part.
Hopefully the pics explain? The bathroom video (see youtube link) also covers how it was made.
2mm PVC.