Replacing a crystal with gasket ridge

Hi there, I'm repairing and old GP observatory HF.

I removed the mineral crystal as it's pretty scratched, I tried using Polywatch glass polish but no luck removing these scratches.

It's 34.4mm and 3mm thick, and I can find a replacement crystal for these dimensions, but the one I removed has a small ridge where an O-ring goes.

I strangely can't find a crystal like this, so I'm wondering if I should 1) buy a 34.4mm non-ridged crystal with an o-ring and it will fit, 2) buy 1 or 2mm smaller to allow for the O-ring.

Here's a picture if helpful

https://preview.redd.it/p96h5pbe87vg1.jpg?width=3024&format=pjpg&auto=webp&s=503989d12f6520f61a833f94660a38bf09caa5eb

Thanks so much!

Leo