I must apologise in advance if this post is 'stating the bleedin obvious', but if your collection is anything like mine you will have numerouse British infantry man / sabotage gas masks missing their lenses.
I have found that by using the clear plastic of clam shell packaging (in this instance from some easter eggs my kids got) I could make some new lenses.
The only thing I had to do was find something to use to draw an appropriately size circle. I used a stencil that we had kicking around (like you do when the wife is a teacher and you have young children). The diameter of the lens is about 13mm.
Sorry I have no pictures, but an easy fix to an annoying problem.