I've watched some brazing videos, but never tried it myself. I like the idea of using copper clad board b/c (1) the FR4 or whatever will give the thing good rigidity, whereas even relatively thick copper sheet will dent if you just look at wrong, and (2) I figure you could make a 90 deg. v-cut in the FR4, being careful not to cut the exterior copper cladding, and fold it like a box - so that the top 4 edges are smooth copper and only the 4 sides would need to be soldered.
Where are you getting your covers - from the usual overseas vendors?
I will check out the T-slots catalog. I really like the Landfall Systems chassis I got for the BeePre -- pretty good value for a quality case. But it would be a lot cheaper to source some aluminum C channel locally and make them myself (even if they don't look quite as nice as the LS ones).
cheers, Derek