Hmm, only other places for a fan that I can think of are either along side the motor on the edge of the chassis, making motor changes a pain in the back side. Or if you're running large enough pinions, on the inside, next to the drive shaft, but I've not actualy checked for clearence in there, and you couldn't get a 40mm fan under the top deck.
A lexan airscoop would be the next step but on the tracks I run at it would just scoop up too much debri thrown up by the front left wheel. Don't see much need for it in our weather though.