We have a birdfeeder that looks like a barn, and you refill it by pouring birdseed through the chimney. This is prone to spilled seed, and a regular funnel often clogs with the larger sunflower seeds. To solve this, I designed and printed a custom square funnel:

It’s square to both create the largest possible opening into the feeder and to allow it to rest evenly on top of the chimney while the feeder is being filled.

The STL and SCAD file are available on GitHub; you can customize the funnel by editing the parameters at the top of the SCAD file.