Entertainment centers, in general are designed to have you cut your own vent holes. The default ones already there are simply a joke.
Cut a couple slots down low in the back. Then cut a couple slots up high in the back. If you need to cover them, you can get expanded metal at a hardware store and spray paint it flat black. Screw it over the slots you cut and it will allow air to move, but make it much more difficult to see through them.
If that doesn't allow enough air movement to keep things cool, you can add a fan at the bottom, to provide fresh cooler air. If you can do it right you may find you don't need an extra fan on the receiver itself.
My 622 is sitting on a glass shelf below the HDTV with no sides and no back. with ambient air temp around 68F, it shows it's normal temp is around 106-115. I haven't checked it since I added the DTV connection, and we have an hour one night were we are recording on all 3 tuners in HD. I bet it warms up a bit then.