Yeah you'll definitely want make sure it can breathe. How tight a space are we talking as far as clearance on the top and sides? Is the back open?
I'm using this
AC Infinity Aircom on top of my receiver and it does a nice job of keeping things cool quietly. They make front and rear venting models also. I picked that one partly for looks too. Its curved front matches the curved front of my Marantz. If you search a but you can probably find something similar for a little less. There are some solutions for ventilating the cabinet as well.
The one knock with those types of fans is that they do suck dust inside the unit. I'm planning on popping the top off of mine and getting in there with some compressed air once it's out of warranty. I have a lot of dust in my area.