After my troubles with cutting the aluminium, I went out and got a got a little set of Dremel cuttoff wheels. I went through about 5 of them, but I was finally able to cut out the hole for the back panel of the motherboard:


I'll need to repaint the back panel to help clean it up, but it looks ok right now. I've tried setting an adapter card into the board, and I've found that the board is sitting a bit higher up than I had originally thought. So now the adapter cards don't fit into the case any more. I'm not sure how I'm going to solve this right now.