My original FFs had the single nylon or polyester strap and a thick piece of rubber tubing as a pad around the Achilles tendon. At some point over the years both rubber pads failed. I think I wound up getting OEM replacement straps too but they didn't come with the pads. Getting the fins on and off was more of a hassle getting the soft nylon or polyester straps up over the strap keeper on the boots without the pads providing some rigidity.
@Stonedfish adding the pipe insulation is a great idea!
But because nylon or polyester straps on my other fins going through a friction buckle have slipped in water I used a safety keeper around my foot too making it a little more of a hassle.
I agree with
@Bruce Baker that the optional padded bungie straps that are available as an aftermarket OEM upgrade with tabs on the top and bottom to quickly pull the pad and bungie over the boot strap keeper and seat snugly around the Achilles without further adjustment are more secure and a lot easier to use.