Diameter of the right hand window on Speedfight clocks?

Hi,

I need someone with a Speedfight to measure the right hand window of the clocks, the one that has the Peugeot logo or the temperature gauge in it.

It's redundant on the air-cooled models, but has a temperature guage fitted in the water cooled models.

I have an idea that it might be possible to fit a miniature rev counter into that window, but I need to know the diameter of the window, to check if some of the ones on the Bay of E's will fit.

This is part of my "SpeedTrek" project and if I can make one fit, then my TKR 2 is getting some Speedfight handlebars and clocks fitted to it, as well as the handlebar fairing. I like subtle mods that need some detailing to make them work. It could also work out a damned sight cheaper than some Koso clocks for the Speedfight bars.