The first cat or small child to come along would destroy your mining operation.
I'm experimenting with 3/4" x 3/4" x 1/8" aluminum angle stock from Lowes. 8 feet of the stuff is about 15 bucks + tax. The pieces can be easily bolted/riveted/tied together to make a cage of your desired dimensions. If it pans out I can order 24 foot segments (shipping is murder for small quantities) and mass manufacture 4 and 8 card cage kits to sell for bitcoins.
I'm experimenting with screwing the cards down on one end of the 'cage' and just letting them rest on a bit of padding on the back end. Biggest danger so far is PCIe extenders falling off the cards, followed closely by cards ingesting cat hair.
That's exactly what I did. I got aluminum angle stock from Lowes. Can't remember the exact dimensions but it was 6ft long. I got 3 pieces of it and some nut/bolts for $14 and some change. My setup is 16 inches wide, 13 inches deep and 9 inches tall. I have 3 5830s in it now but could easily add another 1 or maybe 2.
Here is a pic...
