The title says it.
What would be a decent, structurally sound case to base a mod off of? I work in a hole in the wall computer repair store and on occasion, once in a while, we get an order for a custom built computer. Of the dozen or so builds I have built, all of them have had cases, whether it was Foxconn or Cooler Master or Antec or the such, which were manufactured out of flimsy sheet metal. It seems as though, if I wanted something a bit more heavier duty, I'd have to sacrifice something from a name brand like a HP, Dell, AOpen or something along those lines.
So my question to everyone who's had more modding experience than I, who makes a good, sound case, that isn't particularly made of thin sheet metal?