Enclosing Flsun V400 is fairly easy because of the extrusion slots on each pillar. I have designed top/bottom caps for the side walls and now I'm working on the door and hinges.

How to build the sides:

1. Print 2x the STL and 2x mirrored version (the cap is split in half to fit the V400 build plate)
2. Use a 5mm pin to connect the sides together (the pin should be up to 30mm in length)
3. Get an 800mm by 405mm acrylic sheet (400-410mm will fit nicely into the slots, use 1.5mm or 2mm thickness, anything above will be hard to bend in shape, also it's a lot easier to break the thinner acrilic at home).
4. Push the first cap into the slots and slide to the bottom.
5. Bend the acrylic sheet into the slots and align it with the bottom cap.
6. Slide in the top cap.
7. The pressure from acrylic should hold it in place but you can also use t-nuts to create stoppers for the bottom cap.

I printed and tested these out of PETG and the fitment was just right. I've attached Fusion 360 files for anyone who wants to tweak it.

Changelog:

2022-11-12, v2: improved tolerances for the connecting pin