Home Depot
My local Home Depot sells the pink (dense) foam in 2' X 2' squares - more than enough for the empennage, flaps, and ailerons.
They only had ?" foam so there was a bit of cutting to get the foam down below flush (to apply fiberglass). I've not seen any ?" foam, but if you can find it it would be a lot less work.
Use an epoxy/flox slurry; very little to hold the foam in place.
If you've not assembled the items on the plane, simply press the foam up against the component sufficiently to cause the outline to be impressed into the foam, cut and sand to fit.