I started with an isometric view, but a sectional view was quicker. Maybe I'll try again in Krita instead of Photoshop. The top half of the 2x8 with the 2x4 and posts make a box. The result is a slab with the post let into the edge with a section of stem wall between the posts.
The girts span post-to-post roll insulation would be installed horizontally, you would need three with 2' rolls. If with a rammed earth floor, maybe 2x6s inside and out.
edit:
If you find a deal on a shipping container right down the street, there is this on Boing Boing:
boingboing.net/.../how-to-build-a-house-out-of-shipping containers