This is a parametric lego duplo built in openscad.
I exported and printed a 2x2x1 duplo, but other sizes should work too.
The sizes for the duplo stone could need some more tweaking to make the fit perfect, but current version still sits pretty good.
Either just print the 2x2x1 stl file, or get openscad and generate your own duplo.
You can modify the duplo by just changing the 3 parameters of duplo(width,length,height). All are given in multiples of the 2x2x1 duplo.