Many moons ago, I was a wastewater treatment plant operator for the City. (I still have my Grade 5 Operator's Certificate.)
I also ran a couple of small plants on the side, and still have my lab kit.
On TV a while back, I saw a beer commercial where the brewmasters were drinking out of a couple of pieces of lab ware called Imhoff cones. In wastewater we use those to measure settleable solids, generally in raw influent and finished effluent. Presumably, they are used for a similar purpose in beer making.
Anyway, I got my kit out and retrieved a cone and stand.

It has the exact same top diameter as a Stratus gauge. Calculations say (if I remember correctly) that 2 ml = 0.01".
I think I'll stick one out in the rain and see how it compares.