There is a smarter / easier way....
Will S's first link there, yep. BTW, that seller (starswizard) is Mr. Fred Pauli, who used to own Wholesale Optics but is now semi-retired. His website is astroptx.com and he's got a bunch of new old stock stuff, plus he's a great guy. Bought quite a bit from him, and all of it is quality ..... even have that same filter in .965.
Well.... disregard some of that.. I posted that just as your comment came in. I see you are looking for a narrower notch instead of "broadband."
If your focusers on your scopes can accept 1.25" Baader makes a 1.25 to .965 adapter that is threaded for filters. Then just buy a 1.25 Ultrablock / etc. I believe Agena Astro carries these adapters.
But, you might ask a glass shop.... they might be able to cut a 1.25 down to fit in the .965 filter cell. One thing though, I'm not sure if it would get scratched or damaged somehow in the process.
Edited by telesonic, 26 March 2020 - 08:36 PM.