I saw an aluminum case being sold for the K2. If I wait long enough, they'll probably make one for the 3, but none of the sites or ebay listings had good pictures of it.
It could be sturdy, but it could also be flimsy, since nowhere in any of the descriptions does it mention the thickness of the metal shell, just that it's "Aircraft Grade" which, not being given the actual alloy number, is just a meaningless marketing gimmick. For $15 or $20, I'd give it a shot though.
The one thread I found about a military grade kindle case used a pelican case with a "pick and scoop" foam insert.
I'm actually thinking that will be what I'll do (with a much smaller case, since I don't quite need as much protection) if I can't find anything pre-made on the market.
An mcover and EVA case might also be a good idea but even though the market seems to be for covers, so far I actually prefer to read my kindle naked (the kindle being naked, not I).