Interesting that Edmund said the HA glass could be cut - that would indicate it's not tempered, but it could be cut and mounted much like the round piece I have but better. Copper wire should be soft enough to hold a piece in place without putting much stress on it. The piece I have doesn't lay as flat as I'd like, it needs to be a tiny bit smaller.

I've received a response from Newport Glass - they've asked for a sample so they can check it on the spectrometer and get a good match. I'll send them what I can and see what happens. Might take a week or so for it to get to them. I'll update here when I hear back.