Creativity Suggestions?

Dec 28, 2016
So planning to replace a sight glass window for a bike master cylinder and seeing all fixes from penny to a quarter being used for it. While I order the right part, can anything be used for short term - like any plastic from a bottle or a broken glass or even a penny? Any other creative suggestions? That's how it looks like: Sight Glass Thank you thumbsup
I'd want something flat so the o-ring could seal against it. I guess a penny would be "flat enough" but you could file it.
Not sure the diameter but maybe a piece from a door peephole, probably to go lowes or home depot and see what they've got and see if any would fit.