My unexpertified advice:

I think 80 degrees is the maximum heat you want a ferret to have to endure (ask a vet to be sure). Mount a thermostat on your handlebars and when it gets near 80, duck into someplace with air conditioning. Check the weather everyday, and if the temperature is supposed to go above 80 for that day, don't ride that day. Provide a shaded area for the ferret. Night riding can be a fun alternative. Bring a plastic bladder of some sort; in an emergency, fill it with ice, and put it in with the ferret - he'll know what to do.