Locking strap with zero modification
I have a couple of these and they're awesome. Works on almost every strap button (except for a couple, mainly ones that have their own locking system). Ratcheting locking system is very sturdy, though not quite as easy as some of the push-button/pull-pin systems, though with a bit of practice you can learn to one-hand it.
Zero modification to guitars
Sturdy, reliable locking mechanism
ConsNot as easy to operate as some locking systems
Some strap buttons aren't compatible (though very few)