Need help mounting a cooling fan

I have a 4 to 4 1/2" variable speed fan, similar to the Coralvue smart 4" fan. I have it attached to a piece of egg crate. In the summer months I set the fan, mounted to the egg crate, such that it blows air across the top of my tank (Its simply set at a bit of an angle on top of the screen top - attached to nothing). Works well for keeping the tank at a constant temperature. Problem is that it is precariously mounted. If I happen to forget about it, and go to move my screen top w/o first moving the fan...... you can see the problem - it usually falls to the floor behind the tank (luckily it has never fallen in the tank). I really need a better way to mount it. Help?????? How can I do this??????

 

Here's a pic of the fan - attached to a piece of egg crate. Then the fan & egg crate are set/balanced between the lip of the tank and the screen top - 2nd pic. An accident waiting to happen.......