My wife is very difficult to fit into fins. We finally found that the Avanti Quattro + with the bungee strap fits well and this is a good fin, but she still has the strap slip off now and then during the dive. A smaller fin hurts her foot. Anyone have an opinion of which dive boot has the best "ridge" on the back that will help keep her fins from slipping? As far as I know the fin straps are not adjustable...