Personally I'd cut a hole in the backing exactly matching the XBX, and place it on its side blowing out that hole (so top facing backward). That solves two problems in one, you're ventilating the AV cabinet and removing the largest source of heat at the same time.

Having the bottom of the XBox facing forward might be a bit ugly, though, if you care about that. If so, you could also do the same but blowing in - A bit less efficient (since you're venting the heat into an enclosed space), but should still basically do the trick.