Here's a strap lock that works for Yamaha style end jacks - The Acoustic Guitar Forum
Acoustic Lock Acousti-Lok Strap Lock Standard Output Jack - Esse Music Store
Adding Strap Locks To Your Acoustic Guitar Has Never Been Easier | Acoustic Guitar
2PCS GUITAR STRAP Locks Locking Buttons For Electric/Semi/Acoustic/Bass New C6O0 EUR 3,78 - PicClick IT
Amazon.com: Musiclily Pro Heavy Duty Guitar Strap Locks Security Straplocks System for Fender Strat/Tele Electric Acoustic Guitar or Bass, Chrome (Set of 2) : Musical Instruments