Dymax (I buy a lot of their products, very reliable! though the LEDs could live a bit longer..) make a fan that can be clamped to the side of the tank:
^ That's a great review, and kind of funny also for the appliance abuse...
Bettas can tolerate a surprising amount of heat - I live in Australia and it gets upward of 43C here (48.9C was record heat in my city, boy that was hot..) and an ordinary house fan cooled my tanks okay last summer. I think one of these would really help. But you'd have to maybe devise a way for air flow to occur, without the betta jumping out..