For a billetproof deal I'd suggest you get some thin sheet aluminum (flashing at HD or Lowes comes in a nice brushed finish). With some judicious trimming you could make a nice little bomber style seat. If you wanted to get real carried away, take a router bit of the correct scale and make a die to stamp in some ribs, wood dies would be fine for one or two uses. Even further down the trail you could make some small rivet heads, or look at a craft store for ready made stuff. Glue them on.