Omega Pocket Watch Crystal. 🇬🇧

Not. flat! These crystals came in a variety of heights (as in dome height) which allowed space for the hands, and to allow the cover to close. These can be had in glass (when they can be found), or more typically, plastic. The glass ones are thin and very fragile, and they are not a press fit. The plastic ones are more available, and they are usually chosen in a diameter slightly larger than the bezel, and are shrunk with a crystal fitting tool for insertion in the bezel. I infer from your post that you are planning on obtaining a crystal, and fitting it yourself. All I can say is good luck!

A glass crystal for a hunter cased watch, is about 0.30 mm thick. I wonder about your chances of being able to cut to fit a glass crystal that thin. Buying several glass crystals of varying sizes with the hope of finding that one fits is very chancy, and expensive. Especially so when they break easily. Your watch, your choice!