I have not tried making my own but have had some experience with assembling the light, so take this FWIW.
I would imagine that any piece of silicone with the appropriate thickness would be sufficient.
Find a punch that would give you 1/4 inch circles or thereabouts (measure your existing one to make sure) and start making your own covers.
The real trick is inserting them back in so that the switch doesn't stick. It is more of an issue with thicker material, but you don't want to go too thin as the cover may not seat properly. Sorry, no tips to offer. You need to do a few to get the feel for it.