Here's the trick
Sticky-backed Velcro Tape
If you look at the pic below you can just see it between the fan and the DSI body.
So the Velcro doesn't block any air flow, I cut triangles for each corner.
It works well, and I had the fan running for 15 minutes to test if it falls off. Nope, it didn't.
And if you want to remove the fan, just pull it off, and stick it back onto the Velcro when you need it.
