I'm considering starting with 3 eight foot 4x4's, staggering them so the middle one is about a foot "longer" on one end. Then laying down a piece of prolly 3/4 plywood on top of them.
Then with that base, build walls either with a frame of something like 2x2's with a skin of luan plywood or maybe even just a shell using a boat building technique called stitch n glue. Like in this link http://www.glen-l.com/boatbuilding-with-stitch-and-glue-plywood/ .
I think either of these will let me build something big enuff to put a couple people(kids) in, and still pull it around with the wheeler. It hopefully would still be lite enuff to so that one or two people could put it up on 10 or 12 ft stand if the need ever came up.