I finally finished the breadboard kit, which was probably printed at least 2.5 times over with failed parts due to some problem or fitment issue. I need to get better at this and am learning a few tricks which help, but (for me) designing FDM printed parts that need to fit together can be a bit trial and error'ish. Allowances for parts that just need to fit should be simpler than parts that need to slide or move against each other, and the smallest difference can mean the difference between a button sticking or moving freely. Of course sanding helps, but I would like to avoid that if possible.
This version though, is just about right (there is one minor change I made since but it should not cause any problems). I am gonna print another one to test the STL files which I plan to post up on Thingiverse (or maybe Instructables), then it will be really finished. Hopefully there will be some other folks that will make compatible modules for it since I would like to add some more, but can't think of anything right now.