The capillary boxes I use for my tomatoes and cucumbers have a rather small triangular watering-hole in the corner.
This means that water splashes everywhere when I fill them with water.
So I quickly designed this funnel to fit the corner hole, providing a much larger opening for watering.
I added the source-file in several formats - made it in Fusion 360.
I'm not a pro-designer though, so please excuse me if the project is a bit messy :-)
Due to the design, it can be somewhat difficult to print - My best result so far has been by placing it upright, with the funnel pointing straight down.
This does mean a lot of support is required though, but I don't think it can be avoided, regardless of how it's orientated while printing.
Note This is released with the non-commercial license.
Should you with to use this commercially, contact me and we'll figure something out. I'm not greedy :-)