If you're going to do it the hard way, can you make a gasket punch? Sharpen a tube with the right ID for the OD you'd like the gasket to have. It'll have more of a fighting chance at sealing than handcut.
If you sacrifice a bigger o-ring the diameter you need, roll your own
https://youtu.be/d0uuSNGkYJY
(I believe it's Rubberized CA, rc car tire glue)
'Ave' had a better video on making them, but i can't find it