Go to a company that cuts glass for windows. Ask to them if they can cut tempered heat resistant glass like they use in fireplace doors into that small of a circle. Then you need to find that glass or ask if them if they can source it.
I can't seem to find a picture of your furnace and I haven't worked one like that before, but a type of silaca sight glass is used often on some appliances. It looks like plastic, until you touch it. Popular on Engineered-Air units. It might be easier to find if your current sight glass (or hole) is small enough.
This is an older thread, you may not receive a response, and could
be reviving an old thread. Please consider creating a new thread.
DIY Home Improvement Forum
A forum community dedicated to Do it yourself-ers and home improvement enthusiasts. Come join the discussion about tools, projects, builds, styles, scales, reviews, accessories, classifieds, and more! Helping You to Do It Yourself!