Sportsman's Guide has these and the Hardiggs all the time for pretty cheapI have a couple of these in the back of my 60.
They're tough and have the water tight gasket and I wouldn't think twice about stacking them or standing on them (at the corners) if necessary.
They fit perfectly under my sleeping platform and I got them for like $50 for both.
Admittedly, $50 is a steal so you'll have to keep an eye out on craigslist and such to find a good deal these days.
When I bought them, I think they were billed as a medical supply chest - army surplus of course.