Can I offer some engineering critique, the letters should be proud (raised) versus recessed as the sand will stick in the cavities and not allow the letter to form or pull the letters away(separate from surface) . I like the idea though, leaving a message as to where you have been. Also the letters should a 1/3 to a 1/2 proud as they are wide as so to have enough base so that they are self supporting. Hope this helps. and again great idea. This would be an awesome addition on some work boots.