This was a quick project that was done for a friend.
In order to get progressive curves I used a for loop. I picked 1mm steps which is a little rough, but is a good compromise with speed of rendering. I might play with this later to make the step size configurable. After all it takes much longer to print than it does to render.
The basic idea is that water flowing down the water spout will hit the diverter and spread out instead of just digging a hole in one spot.
Print large side down and the downspout hole up.
use 3 outer layers to make it water tight.