My goal is a weight system that costs less than a dime.
If you're main objective is that it should be cheap, get 2 bits of 6mm or 8mm-ish I.D. tube (depending on your spine dia.) about 40mm long, punch a hole in one side of each about 10mm from one end. Slide the spine in the short end and out through the hole ( like LE fittings in the old days) put the other on the opposite way around so the 2 loose ends face each other. Now find an old 6/8mm bolt, bit of rod, 6" nail etc and push it into the 2 loose ends and bring them together enclosing the rod/bolt.
Cheap and a lot less likely to go AWOL than a balloon full of coins.