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.