The Target insoles on the straight Vibram worked but were quite thick.  The next material was suede.  $4 provided enough suede scrap from Hobbylobby to test on the Xeroshoes. 

Whether or not it lasts, it looks better than any other sandal.  There's also cork & vinyl.  The 2 mile test revealed no problems.  They might get away with no material in front & behind.

Sadly, browsing xeroshoes revealed they no longer sell the genesis & seem to be moving to traditional shoes.