When an older brother/sister likes to build a marble run, it is good if there tiny siblings don't choke on the marble. So I started to make parts for a version with 42mm big marble.

The height of the parts is made so that straight segments can be built by putting regular DUPLO bricks as rails.   

The marble is printed in two half spheres that you glue together.

The big curve have a built in support wall, that you break off after printing to make it fit a DUPLO brick. 

I printed in PLA so far and it is great fun to build marble runs. (When i'm allowed to do it for the kids)