Thanks for the responses. Unfortunately there isn't really an option to put the box in a more "hospitable" location, but to be honest there are two Directv receivers, two big audio amplifiers, various other gear like a DSL modem, firewall, wireless router, multiple switches and other stuff and the stupid cable box is the only one that has trouble with the heat. The environment isn't ideal, but it is what it is. I've already got it separated from the equipment below by several inches, and about a 1/2" above - I can try to improve on that but it is probably as much the fact that the room itself gets quite warm as the heat surrounding it. Otherwise having the box fan blow directly on it would help, but if the air being blown through it is already warmer than it would like it won't help much.
That Samsung GX-SM530CF is the sort of thing I'm looking for, but unfortunately per the specs in its manual it draws up to 25 watts and the operating temperature range tops out at 95F - even less than the cable box! Maybe that's conservative, but I probably don't want to buy one (and a digital audio to RCA converter) only to find out it overheats too. If I can't find anything else maybe it'll be worth a shot.