My bike doesn't leak and I change the oil so often that the site glass isn't really necessary.
It would be cool but Then I would need a ream of the correct size
the glass is not really necessary .. an oil level screw will do the same work and is easier to fit to the cover..
to refill the tranny open the level screw, fill the tranny, till oil shows up through the hole . drain it till only little less oil comes there, put the level screw back in, ready...
i know this from old german bikes .. and boyesen offers aftermarket clutch covers, that have the level screw instead of the glass...