How do I attach a fan to the top of an incubator?

I bought a single 3" 12VDC cooling fan from Radioshack to regulate the temps in my 3' tall wine cooler incubator. I'm sitting here scratching my head on how exactly to mount it to the top without drilling any holes. Should I use velcro or monster glue? Any suggestions or let me know what you guys do, thanks!

Attached mine with screws. Used long ones and found little cylinders (I think they actually came from when I removed the compressor) to out between the fan and the ceiling to aid in circulation. I figured I didn't want to put the fan right up against the ceiling because it would prevent the fan from sucking in enough air.