But yeah, on most horses, if it slids back like that, they will buck like no tomorrow (happened to me once when I didn't realize someone had take the strap off the cinches on the saddle I was using -- no fun!). Ever notice how bucking horses wear flank straps? The discomfort encourages bucking. You've got a very well-mannered boy there :)
Tightening it won't stop it from sliding, just make it that much more uncomfortable when it slides back.
Any small leather strap will work just fine, or even short piece of bailing twine until you can find something suitable. There should be a ring or something on both cinches that you can attach a strap to.
But I would certainly recommend against using a back cinch that is not attached to the front cinch ;)