I use 3M double sided velcro when I have it. It sticks very well and is hard to come off. But like you mentioned, can leave a residue to clean off if moved.

I have tried Gorilla double sided tape, it showed great promise. But for some reason, after a few hours to a few days it loses its grip. This has happened on every job I used it on, on every type of surface. Was very disappointed, I really wanted it to work. Every other Gorilla Glue product seems to work well and quality.

I would NOT use magnets. Depending on where the magnet is stuck to, it could interfere with a memory device or processor. Not worth taking the chance. Plus, you still have to mount the magnet somehow, so you are creating an extra step to just mounting the device with what you were going to mount the magnet with.