Ok I have been playing around with the 5/8 x 3/4 poly tubing. Since the GTUS threads are 5/8, the tube will either slip on or have very little resistance when screwed on. Rather than have to heat and shrink the whole tube, I used a soldering iron with a small tip and at 3 points melted the poly tubing slightly so it caused a small bump. I did the first on the very bottom .010 from the edge, then the next .1" up, then the next .2" up so when you look down on it, it forms a triangle, but as you rotate the tube, each point is higher than the next. This worked perfectly, the top is secured and does not spin and screws on and off from point to point. Let me know if this make sense.
Dan