What about using these line loks attached to the tail, then attach it back to the main line under tension?
This and the Klemheist both have an extra part slightly removed from the UCR bury. One of the reasons for the surgical tubing suggestion was that if the tubing's inner diameter is a mm or two larger than the line then it should slide over it pretty freely.
This past weekend I tried using clothes pins to apply a little squeeze on the running end of my UCR's constrictors. They worked great. I also tried the micro-size of these little spring clips, and they worked just as well. So anything that can provide a little squeeze on the running end of the constrictor will prevent it from slipping.
"Every minute outside ... is a good minute!" -> Calvin & Hobbes, 8/1/1993
Here is how I solved the problem
Make a small continuous loop from cord, I started with 6 inches.
Prussic it onto the suspension line, pic 1
run the prussic loop over the UCR loop, pic 2
Put the suspension line end through the UCR loop, pic 3
straighten cords and snug prussic loop on main line, pic 4
The prussic will slide on the main line and hold tension on the UCR at any point. No metal parts
Works great for my uses, but does lengthen the main line to the tree some
That said, my experience is like amac's -- I've never had a problem with my UCRs slipping if I've made them long enough.