Sorry but I don't understand the reason of DesertFox closing the holes that are outside of the cap's gasket. It seems pointless.
This has solved fuel overboarding for a couple Kitfoxs and also as Lowell stated if the cap integrity is compromised you lose some ram pressure from the fuel cap which may be causing some to experience unequal fuel drain from their fuel tanks. Might seem pointless to some but has helped in some instances. Sure helped my model 3 years ago. Hurts nothing to try even if it has zero effect.
My new caps will get this treatment before first flight. That and the blue Mercedes cap gaskets.