First dremel out the back and front fan grills, it your not to skilled with the dremel you can just cut along the edges instead of making a circle. To give it a complete look you can wire grills (see my pics for exp.)
mine:
Here is how my circle turned out on another case
Next I would cut the lower drive bays out so a 120mm fan can be mounted in for the front intake fan. Maybe cut a fan hole for top exhaust? and depending on what mobo your using (mATX or ATX) cut a small hole on the bottom to hide the front panel/ usb wires, also I slot for the 24pin power cable. Aluminum is pretty flexible and you'd be amazed what can sqeeze behind things.
Or you can also mount a fan on the bottom for intake directly to the graphics card (see my pics for exp).