First I'd try a wrap of tape on the threads, teflon or electrical. If that didn't tighten it up I'd try fouling one of the threads down in the cap but not the first one or two, just do it lightly til you get the friction you need to hold it tight. Another option if you have or know somebody that has access to a shop you can make a sling mount. I needed one for my 11-87 and made one that works fine. If you want to go that route let me know and I'll tell you how I made it.
AGS