The way I did this was to build the frame work outside the van then assemble it inside. This meant I could get a perfectly square cubicle and it was easy to install. The size of the cubicle were made of 18mm ply and the front and back were made from 50mm x 25mm pine which I had cut and planed down from some very cheap CLS. The front and back is a framework made using Mortice and Tenon joints for strength with the back panel being insulated. The front panel wasn’t insulated as half of it was the door and the other half was used for running services.
This side panel was an 18mm ply board cut to size then 1mm plastic glued to it using an industrial contact adhesive. I then used a laminate trimmer with a flush cutter attached.
You can see both side panels here.
You can see the small opening at the bottom right for the toilet cassette.
Dry fitted to ensure everything is flush and fits together correctly. It my first attempt at joinery.
And fortunately I got it right.
I will come back to this job later as I didn’t want to block the back of the van up just yet.