[David]

I've given more thought to a foundation that's strong and portable:

I'm thinking two 16' long 4x6 skids notched 1" deep along their length to accept 8' long 2x6 floor joists, 12" OC, that are also notched 1" to fit the skids. Once finished out, I think it would take being dragged around by a small tractor or fork lifted to a trailer if need be. It would take some extra router work, but it'll keep it almost as low as a 4x4 skid and 2x4 joist foundation.