Recently I purchased a 1940's Invicta tank style watch. It was an impulse buy and for under $10 I figured I could make a buck on the deal. It needed work. It was not running, missing a crystal, and had no sub-second hand. Tried as I might I could not locate a reasonably priced crystal for it. Luckly, I have a shoe box of old watch crystals purchased a while ago of various size all vintage glass. I dug through that box and found one that was close. It at least had the same general shape. And it was only about 3mm too large.
I got my hands on a set of diamond hones from Harbor Freight ($10) and went to task grinding down the crystal. It took an hour or two and I went slow and wet the stone to avoid excessive heat and chipping. The end result is not too bad. A watch looks much better with a crystal than without.