As a temporary fix, or to see if you'll like Stainless Steel Spring Straps (Which are great!) you can go to your local hardware store and pick up a bungee tie down. I cut the clips off, then threaded them as shown in the pictures. After 3 years they have shown no sign of excessive wear, or any slippage. I did find that you need a boot with a solid pad on the heel or the bungees can "dig" into your achiles tendon. Other than that issue they work great and are easy to don and doff.
Click on each for a larger image...