No more difficult than getting glass that has been made specifically for optical transmitance, meaning that both sides of the glass have been polished to within 1/4 wave, are parallel, no inclussions, imperfections, etc. Then you have to cover the glass with the proper coating to give it the 'color' that you want. Then you have to cut it down to the proper size, mount it in threaded holders that will fit onto your ep's. See easy!

Obviously, you can take green cellophane wrap and place it over your ep, but somehow I don't think the end result will be what you are looking for. But hey, at the cost of cellophane, what have you got to lose, give it a try... Clear skies,