I messed around with washers... but I still think the original clear plastic piece is the best\Another idea, less outlandish than my last one. The hardest part of a flashlight mod is finding something that fist perfectly in the lens cap to attach the atty connector to. If you could find something that fits the flashlights you sell (perhaps with an appropriately-sized hole in the middle) and sell that along with them, you'll have taken the biggest hassle out of flashlight modding.
Oh, and it looks like the contest is over