This is a very simple downspout, but it works well.  It sits up high enough that you can mow underneath it.  The vanes disperse the water out, which keeps your lawn from eroding away.  It doesn't kick water very far away from your foundation, but it's a couple of feet and good enough.

This replaces traditional types of downspout extension filters like this from Home Depot:

However, the downside to the long accordion style extensions is that I keep hitting them with the weed trimmer and breaking them.  My redesign spin on this model lifts the downspout off the grass.  The vanes will catch any large chunks of stuff and still allow water to pass by.

This fits a standard 2"x3" residential downspout.  The interior dimensions are 61mm x 77mm, which fits snugly.

Printing

I printed this with just plain PLA and it's lasted for around 4 months in the sun and rain.  It's a little warped, but it still serves its purpose.  If you want to go crazy, ASA is probably a better choice.

I also printed this in super-draft mode, 0.28mm layer height.  It's a little ugly up close, but from a distance looks great!  You can print without supports if you're confident in your printer's bridging performance.  If not, print with supports both on the build plate and inside the model.  The supports still pop off pretty easily by pulling them up through the top spout.

Installation

After printing, just slide it up your gutter.  The tolerances are tight enough that it should stay with just fitted pressure.  But if it slides off easily, use a couple of wood screws in the sides to hold it in place.