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A standard Slinky is 2 3/4″ in diameter with 90 turns. Supposedly, it can be stretched to 15 feet without deforming. The number of turns for a single Slinky would not be enough for resonance in the AM BCB, however, and you would have the ridiculous situation of needing an additional loading coil to resonate the helical antenna. Two Slinkys in series might be OK. With multiple Slinkys, you might have to clip or off turns, or use a tap, for resonance.
The fact that the steel wire of the Slinky is flat doesn’t matter much compared to using round wire. The angle of the flat wire also doesn’t mater much.
There are different size Slinkys, and even plastic ones.