Seriously , once you find some materials , anything is possible ... it does require some “backyard Engineering” and a bit of creative visualization (imagination) , but it is possible to build a foamie (or ? ) camper “on the Cheap” ... The acquisition of materials is the biggest effort... like a “Scavenger Hunt”... put the word out , tell your friends what you’re looking for ... check local sale forums (like craigslist) ... and generally be Aware of anything salvageable ... I can’t pass a trash pile anymore without at least a cursory glance..
The Foam , free or cheap we find
mostly around here is the big blocks of beaded foam ... and being the same density as the Blue or Pink form , is perfectly acceptable for camper building ... and it does shape and sand nicely as exemplified with our paddle board blanks .
Choices of skinning material is all what you can find or have on hand ... I’ve got a collection of old towels and was thinking about using that as a thick skin .... only thing is it may be a bit of a “textured" finish once painted ... a Fuzzy Foamie ...

Old bits of fabric , no matter what color or print you use are all homogenized with a coat of paint ... and speaking of paint , check the paint stores /areas for the “reject “ exterior paint , usually the wrong color for someone, so is sold dirt cheap ... Also check the paint recycling places in the area ...
This has become a “hobby “ for me now (Thriftiness, it’s the Economy eh ? ) ... I’ve even found some really Cool "band clothes" (costumes) at the local Thrift stores... Clothes I couldn’t find or afford anywhere else. I’m into old retro bowling shirts ... must be watching too many Sopranos reruns...

A little Laundering/Dry Cleaning and ... bada bing !!!
