Wherever you mount it, make sure it is on a firm stiff structure, or as much so as possible, and mounted firmly to that structure. As an example, a certified installation of a garmin AHRS module says to mount it on Primary Structure. The goal in that statement is that it is mounted on strong stiff structure, such that it will not be subjected to resonant vibrations which will reduce the accuracy of inertial modules. Although it may work for some folks, I would caution against mounting on velcro. I have designed many certified avionic installations and cannot think of a case where this would was acceptable (except for an ELT that the vendor designed to be retained with a velcro strap system). I know this is not the certified world, but it is good to emulate the certified world if possible. Eric using a nice stiff Z bracket to strengthen a mounting surface is a good example to follow. If you use stainless or brass hardware wave it in front of a compass needle to double check that the needle does not move.
