Great project!

Do you have a screen on top of the funnel to prevent dirt, leaves and bugs from getting in there?

A few "rain collector" designs exist to give you ideas:

https://www.yeggi.com/q/rain+collector/

https://cults3d.com/en/3d-model/tool/rain-collector

OOoo 3D printed Birdhouse & rain water collector:

https://www.stlfinder.com/model/bird-house-with-rain-water-collector-5tdrUBwV/2004085/