John, I had a very similar problem with my Tanglewood 6 string. The solution I came across on another forum was a very simple Dunlop strap lock. They're effective and only cost a few dollars. I use them on all my instruments now.

It's not totally obvious from the picture how it works. The clip consists of two plastic discs each with a cam shaped hole. The inner disc rotates in the other. Turn it until the holes line up, slip it over the strap pin then rotate the disc to close the hole as much as it will go. I've never had one work loose.