..."But I need a material that's waterproof, that I can drive screws into, for spars, and other supports... any ideas? “ ... square PVC tubing ... something we’ve yet to delve into , but seems very workable ...
That’s such a CUTE Foamie’ Teardrop !!!

... George's drip channel idea is brilliant .... the water IS going to get in .. and it just needs a quick path to get
Out ...
Another idea might be to
slightly angle the Galley floor (work area) rearward so that it serves to drain the water back and out of the trailer , but not so much your Tabasco bottle rolls off ... From our humbling experiences with our first wooden TD , water tended to collect in the Galley inside corners and stay there unless we tilted the trailer back a little , then it just quickly ran out ...
Heat bending of the 2” foam material doesn’t seem to be practical for building a one off trailer ... the best bending requires a “form" of some sort and an even heat source ... a large, even heat source... We investigated Heat blankets , but those are Expensive ...

Industrial stuff ...
However the thinner foam sheets ...< 1” thick do seem to bend well with just a good heat gun , and then can be laminated together ... That does seem doable by a home builder ... and as far as kerfing , George’s hot wire kerfing “sled” just seems like the best idea ... We tried the router/jigsaw bit and it was MESSY ..

The little foam bits just starting to disappear in the yard ...

And with the Math in the kerfing thread , it’s entirely possible to get the kerfs perfectly placed and sized to close completely on the inside curve , making a Nice finished look ...

Lots of Prior Planning ...
As KC stated , the simplest things can involve a great deal of effort to achieve Simply ...

So True !!!