I have a couple of those, don't see how it's anything like a spring strap. Just an emergency replacement for a rubber strap.
First, the OP wasn't inquiring about spring straps, but "retrofitted straps" oer "cheaper replacement straps."
Secondly, the Manta Ray strap isn't just and "emergency replacement." It's a fully functional strap, and once in place, is pulled on or off exactly like a spring strap.