Fred, a few times I have come very close to enclosing my underbelly for winter camping and then have always abandoned the project as too difficult.
The problem with foam sheets is that unless they are protected with coroplast, the foam will crumble. Some folks have used 3M spray adhesive to bond the coroplast to the foam.
And then you have to support the sandwich of coroplast and foam.
Some folks have installed aluminum T-molding to hold up the sandwich. Others have used plumbers pipe strap to hold it up. In either case, you would have to drill holes into the underside of the chassis and then use self tapping sheet metal screws to go in to those holes. Alignment is tricky.
If I lived in a colder climate, I probably would have done this job by now. But we only do a few days of snow camping each year.
Let us know how it goes!!
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)