I've been looking at a lot of different methods for these, and ended up trying Just Jeff's method. I had the extra slingshot band and guyline cord lying around, so I gave it a go tonight.

I got the first side all tucked in, wrapped up, and folded over (though it was a bit of a challenge working with the tubing). Then I got stuck on the the second side. Apparently, you're supposed to push slack back into the tube from the second side (to give the tube room to stretch, but not break). For the life of me, I couldn't figure out how to push the string back into the tube (maybe tube was too small or cord was too big). Plus, I couldn't figure out how I was going to push the knot into the tube! On the first side, I pulled on the string through the other side to get the knot into the tube, but how is this possible for the second side?

Anyways, I gave up on that methodology and switched to the one outlined in an earlier Just Jeff post. However, I couldn't get my clove hitches to stay under tension, so I switched both clove hitches to Common Whipping, and they seem to hold up very well under tension.

Bowlines on both ends: one to girth hitch to tarp grommet, the other to clip my guyline tensioner to.