If you shave the top and bottom of the door so there is a 1/2 to 3/4 inch gap when it's closed, convection should take care of any heat buildup.
If the drive is actually outside the manufacturers recommended range, then yes, I'd put a heatsink on it. Otherwise, change the global disk alarm temp to match the manufacturer guidelines for the hottest drive, and bump all your spinner drives down to the range you want to see.
Ideally you should only get an alarm when something is truly wrong, like a fan gone bad or somebody blocked the closet door.