OK, I'm ready with some pictures of the seatbelt sling(s) I made tonight.
The equipment (seatbelt, scissors, mason's twine, lighter, camping cord) I also used a titanium marlinspike, not pictured:
The Multiple-Overhand knot, which I used for what I will call the "nugget knot," which goes inside the M-fold:
The Multiple-Overhand knot, tightened:
The Multiple-Overhand knot nestled in the M-fold:
EDIT: NOTE: This photo depicts where the nugget knot will end up eventually--not where you should have it when you begin tying the whipping.
In this picture, the "nugget knot" is snugged-up to the M-fold. Actually, you should pull the nugget knot out toward the center of the pouch, leaving a bit of length of the cord, so that you don't end up tying the whipping around the nugget knot. The idea is to tie the whipping
tightly around the M-fold with the cord--not the nugget knot-- inside, and then pull the nugget knot so that it backs up against the tightly-held M-fold.
Setting up the whipping to cinch the M-fold:
The whipping, finished but without the ends melted:
The whipping, completed:
A finished sling:
The pouch with a foam golf ball loaded:
The pouch with a foam golf ball loaded (note, pouch is turned inside-out):
I have not taken the sling out for testing yet. I have this to observe about the pouch, though:
The pouch does not exactly "cup" the ball. What happens is the back of the pouch simple folds, while the ends of the pouch (at the whipping) have their own cupped shape which actually is what is responsible for retaining the ball; but the retention will be effective only while the pouch is held closed prior to a throw or by the centrifugal force of the throw as it is spun.
I have not decided (since it will depend on testing experience) whether it is better to use the pouch "rightside out" (with the nugget knots on the inside of the pouch) or "inside out" (with the nugget knots on the outside of the pouch). When I began the project I was thinking it was a no-brainer that they'd be inside, but now that I have seen the sling in my hands, I think a good case might be made for putting them outside. The edges of the pouch do different things depending on which way you close it.
If you have questions, let me know.
edit: Just saying thank you again to Aussie for the excellent idea and tutorial.