If you think about it, 5-10 minutes of moving a 100 lbs+ robot is a lot to expect for a motor from a cordless drill.
Also, it goes far beyond the length of any match you'll be in.
Back in '99, we had all sorts of problems overheating motors during the 2 minute matches (so much, in fact, we could change out toasted motors and speed controllers in under 2 minutes), so in '00 we designed our aluminum motor mounts to clamp around the body of the motors. This worked very due the ability of aluminum to sink away a great deal of heat, not only to the clamp, but to the rest of our frame. Just ask anyone who welds aluminum and they'll tell you the same. It’s also the same reason computer heatsinks are made primarily from aluminum. They worked even better when combined with active fan cooling, but were far less effective without the 'heatsink' mounts.
My advice to you is to design an aluminum clamp that connects to the body of your motors as smoothly and tightly as possible (for the best heat transfer) and then to your frame and if that isn't enough, fan cool the clamps/motors.
In terms of computer heatsinks, blowing across the heatsink almost always yields better results than sucking air over them. The opposite only seems to be true for very large heatsinks that are designed to suck air via special ducting (name anything produced by Alpha Co. Ltd.).
Adam