The funnel is a LARGE plastic funnel, 380mm in radius, or 760mm across, OR 0.76 meters wide, in other words it fills most of the tile it’s on. This isn’t a random kitchen or automotive funnel you stick in a can, it’s purpose-built for catching rain.
The makeshift funnel OTOH is exactly that, a small plastic funnel made by cutting and unrolling a plastic bottle.
All the pertinent calculations are spelled out in detail in weather.cpp in funnel_charges_per_turn() and funnel_turns_per_charge.
Now that I take a look at the stats, the funnel item is very misleading, it should be pretty light, but I hadn’t noticed that the volume is massively off, it should be 10+, not 2.
Feel free to add more funnel-type items or clarify the descriptions, but I don’t see any issues with the existing items. If you want to add a chance of things blowing away though, the funnel is a great candidate for that due to it’s surface area and low weight.