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You can attach the floor to the foam using resin, and probably a single layer of CSM, or a thin layer of mash, between the foam and floor. I don't think it makes much difference, if any, whether the foam is used to form a bond, or resin/CSM/mash. One thing to watch out for is creating an air pocket between the foam, resin/CSM and the floor. If you apply a small/moderate amount of mash (1/8 inch?) to the back of the floor or on top of the foam, I think it will work. I am actually going to try this this weekend. I will likely use CSM with a couple inches tucked under the edges and mash throughout. I think it will add at most 10 pounds.

I was told by a pro that the step that really matters is the seam along the edges of the panel. Attaching it to the foam is mainly about eliminating voids and getting adhesion across most of the panel. And if attaching the panel only goes so well, you can fair it to what you want. But the seam is really important to get right. i.e. bevel the seam wide and use several layers of glass, e.g. heavy stitch-mat.

Its a learning experience. I figure the more I do on this one, the easier the next one will be.