I'd go with a piece of lexan, that's what I've done for my scopes that don't have kill flashes that fit.
Home Depot carries sheets of lexan, the 11" x 14" is only $13. If you have a hole saw that's the size of the contour then that's perfect. If not then I use a dremel to shape mine. I cut to rough shape them use a grinding bit to fine shape it. Then I just use electrical tape to attach to the front. Works perfect and lexan is perfectly clear, can't even tell it's there. Plus with this sheet you get a whole bunch of them!