Here's what I attempted and pretty easy with no case mods at all. You will need:
USB cable: length depends on how you run your cable, but mine was cut down to about 3 feet.
3 computer fans. Not sure what the amps mean, does higher mean spin faster? Size, whatever is the most common or what you have lying around.
Something to strip wires. I used an Xacto.
Electrical tape.
Something to secure the fans to the Xbox. Zip ties or double sided tape should work.
I cut the a 6 foot USB extension in half keeping the MALE end, stripped the shielding to reveal 4 wires red, black, white and green (your colours may vary). I cut down the white and green wires because you only need red and black. I took 3 old 12V computer case fans I had lying around and basically connected them red to red and black to black to the USB extension. Taped up the exposed wires. Don't need the yellow wire if your fan has one. I plugged it into my old computer first to make sure there wasn't a short. Fans came on, albeit rather weak, they still moved a decent amount of air. 12V fans run slow at 5V

Anyway, I placed 1 fan on the grill in front of the hard drive and the other 2 on the opposite side, more towards the front of the Xbox. All fans blowing in secured with some 3M double sided tape. I plugged the USB cable into the back of the Xbox and they came on, even though the Xbox was off. Bummer, not what I had in mind, but this might work.
So I tested this ghetto setup and after an hour of NHL08, it was too hard to tell if the temps came down. I'll need a temperature gun to confirm. The "bonus" is, after I shutdown the Xbox, the fans continued to blow air into the case! I'm thinking that SHOULD help cool it down. But the fans will never shut off unless I unplug the USB cable. Might need a switch or a timer or something, but I'm not that technically inclined. Unless leaving the fans on for long periods of time won't do damage to the Xbox. I've been leaving my Xbox on at the dashboard for about 5 minutes and then leaving the USB fans connected for about 5 minutes after shutdown. I think the longest I left them on is an hour cause I forgot about them.
I might even try this with 5V fans to see if they move more air.