Why not just mount it the right way?
Not to a rifle but once using two little notched wood blocks as spacers to allow for the over sized head of a silver Ray-o-Vac flash light some buddies and I duct tapped a Ray-o-Vac to the barrel shroud of a S&W Model 76 SMG.
Guess which NRA president gave us that idea.
Worked OK for the time period. We even used that center dim circle shadow of the bulb as an aiming point. It appeared to be minute of infected luddite Zombie at 20 meters........
-kBob