I actually have used Wedi in a couple projects. I am not an expert or professional but will share what I know.
The stuff is essentially sturdy foamboard and can be cut with a razor knife, no power tools needed. It is critical that the entire system be fully sealed and waterproofed, with all joints sealed with the provided wedi sealant. (I used the stuff provided in caulk tubes) While you could cut a V and bend the board, I would not trust it to be waterproof and you would need to seal it with the sealant anyway , so it would probably be stronger to just do a butt joint with sealant.
While I would generally never recommend on planning on anything being accessed from inside the wet area, wedi sealant might work to your advantage because you could probably cut out the box along a sealant joint, replace the part, and then re-seal the box to the location with new sealant.
I’d check the wedi documentation for the correct joint details. I’m not sure exactly where your box will be and if it will have standing water against it, but if so the details might be different or even not at all recommended.