For several years I have been on the look-out for a sight gauge and I even contemplated making one up from spare flowmeter components that I had lying around. However, when I met with Colin from NB Intrepid I discovered he had purchased some small brass sight gauges from Tony Redshaw.
I did the same this weekend and mounted one today. The gauge is certainly an improvement over the wet finger technique and polished-up it looks the part.

Brass Coolant Head Tank and Sight Gauge