Dude,
If you can get 3mm acrlyc sheet you just cut the 2 parts out to fit each.
2mm is too thin, 4mm too thick
Get several spoons from your kitchen and look for one that just about fits inside.
Using tongs, hold the cut part (not too close or it'll burn), over a flame or heat it up somehow until it starts to warp under it's own weight.
Then take this molten sheet and use the spoon to press the shape into the eye.
Leave it cool and bingo! a lovely curved lense.
There is a trick to gettign this spot on so you can see out of the part you look through but the rest is bubbled.
I do have this as tutorial on the UK Garrison boards or you can mail me and I'll mail it over to you