Halcyon spring straps are aftermarket straps that will fit most Jetfin style fins that have similar strap attachment points.
They consist of an appropriately sized stainless spring with an attached cable inside to prevent overstretch.
They are covered with branded round webbing to conceal the spring and prevent abrasion. If anything can be said to be these spring heel straps can! They come in four sizes to cover the range of most peoples feet.