Single glass would be ok as long as there is no risk of impact and the diameter of glass is small - they did that for decades in machine design .... but anything flat inside the brass things are eventually going to get cruddy and unreadable (although glass will be better than plastic, and you could take it out and clean it with acetone or something if it becomes unreadable - no doubt some years down the road) .... it would work (will be interesting to cut a small circle, though) ... I was just thinking that if you could improve it from a 'service-ability' standpoint, it might be worth a few extra bucks.... taking them out and cleaning them periodically is also an easy thing to do.... do what works best for you. There should be minimal pressure against it (vented lube system?). I would have more concern about plastics in the context that they can/do chemically react with anything that even closely resembles a solvent, so choose carefully .... Anyway, enough of this .... just trying to give you some ideas for options so you (hopefully) only have to do it once! Take care and good luck! Hope this helps your "idea file" a tiny bit!
