I have used velcro to secure my temp unit for the last 4 yrs w/ no problem , also used it to help secure a pc of driftwood to the back of the ap cage when I was using it. Never had one bit of problem. I did however use industrial velcro which you can find in the electric section of Lowes/Home Depot. It is very strong velcro wise ( I use to have to use two hands and some might to pull it apart ) and adhesive wise.
I don't think I would use it for rocks.