I did the same with a pair of mine. originally I had them on my drysuit fins but the springs where not quite long enough so I swapped them onto my wetsuit fins and they worked perfectly. If you decide to to make these here is a earlir post describing how to do it and what to use if you run into the not quite long enough problem like I did.
http://www.scubaboard.com/forums/accessories/96114-ss-spring-heel-straps.html
The other thing that I'm not if is in that discussion or another one is the recomendation to buy shorter ss steal screws to go with the shackles. Place the shackles on the fin, insert the screws into the shackles then take a pair of pliers and squeeze the end of shackle so that you tighten it up on the post. This help ensure that the shackles won't come off (though I'm fairly confident they won't, but it's best to be safe rather than sorry). Then replace the screw with the shorter one and locktight it into place. MAKE SURE YOU HAVE A SCREW IN PLACE WHEN YOU ARE TIGHTENING UP THE SHACKLE. Otherwise you run the risk of bending the shackle in a manner that will result in misaligning the holes in it and you will never be able to get it line up again. some people also peen over the end of screw like a solid rivet, I personal didn't think it was necessary but it is an option.
that being said I got a pair of xs scuba spring straps for my drysuits after the same thing that happend to you happened to me. So far they are working great, you can tell the quality difference just by looking at them compared to the cheap ones.
Hope this helps
