Framing with ground contact rated treated wood (pwf -pressure treated wood foundation) woul be strong and hold up for many years. The stuff with the little sipes or slits. Fence posts are usually PWF material, used to be able to get 3/4" pwf plywood, but it is getting harder to find.
Closed cell rigid foam would work great and be very easy, if a little messy to install. Expect an unpleasant dusting of statically charged little beads about the area. Foam is likely very expensive material wise, but would be mich less labour than anything else. If you could get enough old fashioned packing foam that would work too, but most peanuts are starch based now and lot of the newer stuff os injected open cell foam, neither of which will survive very long wet.