There are a lot of aspects brought up that I hadn't thought about, and I'm glad I posted. My idea was to have have buckles and straps installed on the bottom, but design them to mate with sandals that already have buckles on the ankle; then all that would need to be done to attach them is to open up the ankle straps, snap them in place, and use a cord or thin strap to anchor the toes. I think it would take less material to do that then to have a full set of straps.

Of course, it would become pointless if the sandles wore out and the manufacturer changed or stopped making them...

I have an older pack with sleeping pad straps on their base. I think I'll try using it with footwear lashed on and see how I like it.