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.

For the struts I used Arbor brand PVC decking from Lowe's. I glued them in using two glues simultaneously: 5 minute epoxy, to tack things in place selectively, in addition to 3M brand 5200 marine adhesive/sealant for the real hold, which takes 24 hours to fully cure but is water and vibration-resistant, unlike epoxy. Marine products tend to be the best choice for old trailers in my experience. For PVC-to-fiberglass, or anything really, 5200 glues better than Gorilla, more permanent, no foaming, and it doesn't get crumbly with age like Gorilla. Scuff up the edges with sandpaper and a Dremel before you glue, for maximum adhesion.

Hi, sorry I appear to have missed this earlier.

I just finished my alteration. I basically Dremel-ed down the bottom edge of the icebox opening, to create a taller opening. Then I glued in struts on the bottom and sides that I could attach paneling to.

I try to avoid wood for things that are going to touch the trailer bottom. Leaks are inevitable. Mildew smell is the enemy. Moisture will wick up. I have learned to think very defensively about this stuff because no one wants to hang out in a stinky trailer.

Icebox - by coincidence; I'm just about to remove our icebox and replace it with a cabinet or drawers. I'd be interested to know how your conversion turns out (if you don't mind my cloning your ideas). Thanks