Boot Cuffs with folded top and button. This is knitting pattern, easily changed to crochet. I like the folded top cuff with button for a different look but I think I'd make these a bit longer as a boot cuff.
If you're curious how to crochet on flip flops, this post will answer all your questions including if they fall apart over time. Great free tutorial on how to get started crocheting shoes and boots for spring, summer, fall, and winter.