A few hours ago, I made an Aussie Seatbelt Sling (ugh, I'm not going near that acronym!
) but since I haven't yet obtained a seatbelt, I used a section of denim from a pair of old shorts that were too worn out in the seat to wear anymore.
It came out quite well, but I don't really trust the weakened denim for bearing much strain, so this one probably won't see use. I do think it is capable of slinging golf balls, though.
It serves as a working prototype, to let me know that this is a pretty easy design to work with. And I used my glow-in-the-dark cord for both the lines and the whipping.
It occurs to me that the same design could be employed if you used a leather pouch instead of the seatbelt. Since leather would probably not lend itself to being "M-folded," you could make a sort of "H"-shaped template for the pouch, where each extension of the H is essentially a tab about an inch long and about 2.5 times the width of the diameter of your cord. Then you could (as I've described before) tie each line with a bowline or other loop at the end, and sever the bottom "U" of each loop, leaving you two attachment points for each line. So you would do 4 whippings total. I think that if you made the "H" shape actually a bit of an "X" shape, the pouch would cup the ammunition better.
If I get a chance, I'll try to draw up what I'm visualizing, or maybe just cut it out of denim and photograph it so you can see what I mean if the description doesn't do it for you.