Stuart - There is indeed a gasket that goes under the lense. You can get them from any of the vendors or make your own. I used some 1/8" sheet foam that I found at the materal store in the crafts section. It comes in 8 1/2" x 11' sheets, any color you want. It's closed cell foam and works well. I think I had to use two layers. It might cost you $0.50 for a sheet.

George