Mike,
Since you have the 2x's on hand, just build the frame. You aren't doing elevations and detailed terrain to require the additional work of the joists.
Perhaps double up on some of the adjoining sides and build separate sections that bolt together for easy disassembly.
The decking can overhang and permit shaping with a jigsaw and attaching a Masonite fascia board.
You may be able to get away with one leg of a doubled 2 x 4 in the center with the perimeter attached to wall studs. Should be solid to dance on.
