I had been using the “diopter kit” from Fat Shark but it only comes in -2, -4, -6 and I am like a -5 so it helped but never got it quite right. I thought about cutting my own glasses to the right size but never got around to actually doing it. Apparently other people have taken dremels to their old glasses and gotten something acceptable for them. I ended up using http://rho-lens.com/rho-lens-fpl/ and quite happy with the results. Images from rho-lens:
