Generator for any circular Duplo brick. Not 100% accurate compared to original but fits thanks to the original generator on which this is based.

It is very CPU intensive. Depending on size and number sides of the main cylinder.  With the 6x6 size and 256 sides even my Intel i7-9700K running at 4,75 GHz takes several hours to do final render. So there are some exported common bricks in STL:

2x2x0.5
4x4x0.5
4x4x0.5 - slightly larger radius of top studs
6x6x0.5
6x6x0.5 - with added ribs for easier no support printing