My wife asked me to 3D print a bucket she could use for her pendulum painting set-up in the garage.
I utilized the funnel customizer by @aaelberto7 and just created a very large funnel with a roughly 3.5mm hole at the bottom. This design allows for optimal flow of the paint downwards and out while it swings.

I also built in a second handle. You'll want to tie a strong rope from each handle to create the attachment necessary for your pendulum.