ok, i work in a bosy shop and have 5 yes exp in taping vehicles for paint.
do what we call a backtape. this might be hard to explain. your pc fan has 4 sides, and is about what 1/2" wide at most? take some 2" wide gorilla duct tape, cut 4 pieces as long as each side of the fan. take each piece and tape each side so the sides of the fan are covered and you have the rest of the tape overhanging. do this on all 4 sides. now slide fan into hole you made in your tent. now the overhanging tape, lift up against the tent, the sticky side should be away from the tent. now take another piece of tape and stick it to the sticky tape on the fan (from step 1), and stick that to the tent. repeat for the other 3 sides. then use more tape around the corners if needed.
if you like, i can snap a few pics of what im talking about if it might be easier to understand. pm me your email addy.