One of the sites I regularly check is Arachnid Labs (a.k.a. Nick Johnson) and I noticed that he's recently released some dinky little stamped metal angle brackets for laser-cut boxes. Although I've no immediate use, they were too interesting to pass up, so I got a packet to play with.
They're made from 2mm-thick stamped stainless steel, with two M3x0.5 threaded holes and a little bent-up tab to stop the bracket rotating during assembly. The tab can either locate into a cut slot, or you can raster-engrave a recess to avoid the tab showing from the outside of the panel. Here's photos of the brackets and some test pieces:
And a quick video!