yep 3rd design is best, it won't dip lower you can keep it steady, if worried about it sliding around then you have a strength or weight imbalance(!) ... you could loop the tape through the top carabiner in a knot. You don't need sliders if you know how to tie a tape knot or a figure 8, you should prob. get the tape from a climbing store, and finally, it is not a bad idea to have the pvc pipe setup with several options, e.g get some 2" pipe and put it on its own tape, and some 3" pipe, in other words, so you can swap them around with longer/thicker pipes.
hope that rambling helps - i have a similar setup. pvc pipe is great stuff!
make safety your number one priority and don't fall on ye face