Pat,
I read some reviews that mentioned damage to guitars when installing the braces required for this device. Can you comment on how hard that installation was for you?
the part that goes inside the guitar is a magnet with 3 or 4 legs (don't recall exactly)... double backed mounting tape on the legs holds the inside piece from moving around. You can locate it wherever you have room. Easy install, you don't even have to take the strings off if you can loosen them enough to squeeze the inside part past them.
The outside piece (the interface) is held in place by the magnet inside the guitar. It is a very strong magnet that has no problem holding the outer piece thru the wood of the guitar. Sliding the interface while the magnet is pulling it could easily scratch the finish, so they recommend prying one edge up to break the magnetic bond before repositioning anything.
I think the magnet is so strong that if the wood is brittle it could crack the wood or the finish. I kind of wish the magnet was a bit LESS strong.