What a good use of free resources! And if you have a little bit of a slope on it end to end, even just an inch or two, you can probably get some rain catchment off the roof as well!
David Wieland wrote:...(which can even be insulated with foamboard such as Styrofoam).
Does ground-contact foamboard hold up ok for you? I thought the pink/purple foamboard insulation was impervious to nature and left some pieces of it leaned up against a wall with their bottom edges on the ground, and something termite-ish ended up turning it into a bunch of cool-looking tunnels and uncool purple plastic powder.