Another way to solve the issue is to switch to Evans coolant.
Since the system is no longer under pressure, there is not trick to finding the right coolant level. It just is what it is. I've actually ran mine without any reservoir at all.
When I bought my bike, it had a 3 oz brake bleed bottle mounted down around the kick stand as an overflow/reservoir, It was just zip tied on.
Unknown to me the bottle cracked (cheap plastic), I rode it for miles with no problems like that. The fluid level in the radiator was fine when I checked it.