I printed and used jtronics rain collector last summer.
https://www.thingiverse.com/thing:4596000
It worked pretty well but when opening it up after the season I found multiple debris diverting arms broken.
This is a printed insert that aims to drop in place and “fix” the broken arms. It is easy to print makes the rain collector serviceable. The aim is for the down pipe to sit just outside the lip so water running down the sides gets collected.
It should scale to the bigger sizes, but I have not tried it. Going smaller might make the walls too thin.
Enjoy
The author marked this model as their own original creation.