A single strand of 18 guage copper wire.
Stip off the jacket if it has one, fold it in half, and you're done. It's small enough and stiff enough to push through 7/64 amsteel without even bunching it. I've made two sets of whoopies, and adjustable ridge lines with the same piece of wire and it shows no sign of wear whatsoever.
This stuff costs next to nothing in bulk. Better still, it can be salvaged from old phone cord. The small round 4 conductor stuff (known as JK) works best. Newer CAT3 and CAT5 is 24 gauge, so it's quite a bit smaller. 24ga is less stiff than 18 and it might not hold up to all the pulling. If you know a phone guy ask him for 3-4 feet of it, and he'll probably give you 3-4 dozen yards of it instead.
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