A rain gauge shaped like a megaphone is not the standard shape meteorologists use, but it should work if it has the correct ratios between the cross-sectional area at the top and the part of the gauge that actually collects and measures the water.
If this gauge was a straight-sided cylinder that did not narrow toward the bottom, it would be very difficult to measure small amounts of precipitation.
It would require the bottom inch, for example, to be divided into 100 separate markings for hundredths of an inch, and most rainfall is much less than an inch.
Meteorologists commonly use a gauge that consists of a small straight-sided cylinder within a bigger cylinder. The rain is collected by a funnel at the top of the smaller cylinder, which is only a fraction of the diameter of the bigger cylinder.
The inner tube is open at the bottom. The markings on the outer tube reflect a ratio of ten to one, so that an inch of rain would reach a mark ten inches from the bottom.