I think you're overly concerned about the ferret overheating. If its crate is well-ventilated and provides some shade then I don't think it would be a problem. Animals have the good sense to rest when it gets too hot and with the cooling breeze created from your biking speed the crate should be a pretty comfortable environment without the need for any AC unit. It's people who have problems with overheating since we insist on doing strenuous exercise during the hottest part of the day.