How long do you plan on driving? You would need a HUGE reservoir to do any substantial trip.
Installing AC would be the best option.
Alternate options:
Cool-shirt (DIY setup info all over the lemons forums:
Summer Approaches : share your best cool shirt/seat setup links (Page 1))
Cool seat. IIRC someone on the lemons forum did a cool seat as well. Just took the cover off the seat, pushed some copper tubing into the foam, and put a little bit of foam on top. This could easily be converted to a warm seat in the winter as well
(The above was with a fixed back seat which is well insulated on the backside due to the fiberglass construction)