I don't know about the Surefire E2L per se but I've fit a few of my lights with rubber or plastic caps to protect the lens or to prevent them from turning on accidentally in a toolbox or bag.
There are lots of things out there once I get my head wrapped around looking for them.
L to R: Old 7 LED TEKTITE dive light w/small spray paint can cap to protect window, ELEKTRO LUMENS XM-3 w/rubber cane tip protecting switch and plastic chair leg tip over head, Nightcutter Sport with plastic chair leg tip over head.
I also drill a very small hole in any cap I use for a lens cover so I may easily make sure that the light isn't on when I stow it.