Another option is the 3M VHB double-sided tape Streamlight includes w/their pressure switches.
Not as thick as Velcro, I've been using it on my rifles for years (and will probably be using it on my new 940 Thunder Ranch when it arrives this week) and have yet to have the tape fail (ie: the switch didn't come off until I physically removed it from the tape). You can get sheets of it off Amazon, then trim to size.