You don't need to remove the cover off a Mac Mini to cool it.
They're designed to run 24/7 at 35C ambient.
Well, based on my own experience, with the cover on the internal fan working much more, sometimes at top speed and the aluminum body gets very very hot, however without the cover and with the extra cooling the internal fan works only occasionally and the body remains only a little bit over the rooms temperature. Of course we are talking about long runs, in the past with a regular use of maybe 6-10 hours a day I only notice this kind of overheat and noisy fan when play for a while games with heavy 3D graphics.