Duplo compatible bricks in several sizes. The supplied STL has 2x2, 4x2, 6x2 and 2x2 (double height) bricks. The SCAD file allows you to set your own preferred dimensions very easily.
These printable Duplo compatible bricks have an internal support structure, which facilitates printing without additional supporting materials. Also, the central cylinder is notched for increased flexibility with plastics which are less elastic than ABS.
Getting the dimensions of the studs and ridges right for compatibility with original Duplo bricks took some tweaking. If you find that a slight adjustment of the dimensional parameters (specifically ridge width and stud radius) gives better results, please leave a comment.
NOTE (Sept. 18, 2018)
While the bricks work fine for me, some people seem to be getting suboptimal results with them, which may be due to possibly inaccurate dimensions of my design. Give it a try with a single brick before printing a whole bunch of them. For more information, see the comments.