Couldn't you build a flatbed for less? 2ea 4x10 pieces of Plymetal swingdoor material would be maybe $240. Plus some wooden joists.
My friend built his eggshaped camper back in the 70s. It appeared [apparently] on page 92 of Shelter (Shelter Publications ISBN 0-394-70991-8). That page was torn out of my copy.
He started with plywood ribs with diagonal wire bracing, laid on 1x2 stringers and then shingled the whole thing with hand tapered shingles. The foam was sprayed on from the inside. It looked great and he lived in it for a number of years. It still looked good after being perched on a hillside, abandoned for years.
I'd suggest fish-scale shingles, laid horizontally.
The closest thing I can find is this:
...only backwards.
