After letting the bases dry I found two problems. Gorilla glue did not fasten the plastic bases to the paper disk very well. Also the expansion of the glue lifted up the plastic base to the point where the trees were noticeably leaning to one side. I primed the plastic base with the Loktite All Plastics primer stick and glued them down with Gorilla super glue instead. After the glue set, I tinted the joint compound with earth brown and built the sides up. The results thus far are below.
Part 3 soon.