ive actually got one of those, mine at least is a bit too small, and you can see those ribbed lines on the inside of it.
Have you looked at the Plastruct Elliptical or dish domes? Might be able to glue it down to some flat stock to create the lip.
nope, on the one ive got, the ribbed lines are on the inside lip of the lense, youd have to chop it off. Ill try gettin a shot of mine tomorrow.