The PVC decking mills a lot like wood. It's basically foamed PVC. You can rip it on a table saw, etc. It holds stainless steel wood screws very nicely.
There was a TON of test-fitting involved. The walls of the dinette are subtly tapered. The exterior surface is of course curved. Getting from those curves down to a plain old box with walls was a LOT of work, and a ridiculous amount of walking back-and-forth, making marks and sanding.. Sort of like framing out a square inside a circle. Having some wide-mouth clamps is really helpful.
And you will need a level to get the verticals square. Don't trust your eyes the way I did at first.
Hope this is helpful to you.