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.
They are actually a lot more sturdy then they look. Those extender cables are very stiff and tend to hold the cards fairly well. If those cards were to dominoe to the side nothing would happen and they would keep mining. It's possible a cable would pop off but no permanent damage. I have 4 rigs setup similar to that and knocked them over a couple of times. All is well.
And who the hell has random children + cats show up?? I sure as shit don't.