The little knob is always coming out on my flip flops so I designed a thing to provide some support.

UPDATE: For a second there I forgot that you can't deposit plastic into thin air, so v2 is an alternate version that won't be as comfortable, but it should print and work just fine.

FINAL UPDATE: I have actually printed and am using v3. It works, so this is no longer a work in progress

On the underside of your flip flop, slide the strap into the slot (it should be tight) and then push the knob into the hole. I've included a picture of my pre-prototype, made of MDF. ;)