I don't have to transport my balls often but when I do (insert Dos Equis commercial here) I have something similar to below to transport them. From my understanding a snake getting too hot is more of an issue than a snake getting too cold, it shouldn't be an issue having your a/c on.
http://www.walmart.com/ip/Sterilite-...ndingMethod=rr
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I like the idea of a non transparent tub for transporting. Would the a/c really be that much of an issue for a 40 minute drive? I wouldn't have it on full blast with the windows up, I just wouldn't think it would much of an issue for a relatively short drive.