I believe we used Pellon’s Peltex Ultra-Firm Sew-In stabilizer for our prototypes. (https://www.joann.com/peltex-fabric-stabilizer/7477532.html or https://www.amazon.com/Peltex-Ultra-Firm-Sew-Stabilizer-15-Yard/dp/B008I68JYM/)
Similar sew-in stabilizers would also work. If you’d prefer to iron in a fusible stabilizer, I would recommend removing the seam allowance on the heel stabilizer pattern.
For shoes made from an already stiff material, such as denim or canvas, it’s possible that a heel stabilizer cut from the same material as the rest of the shoe could provide sufficient stability.
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