I liked aspects of a bunch of the models I saw out there, but didn't find any that checked all the boxes, so I made my own. 
I wanted them stackable, I wanted a secure middle join, and I wanted them to be open for others to modify.

As yet just the two sets-- a basic top ring, middle join, and solid base, and a set with a pattern of 10mm holes.
I'm planning more variations, and if you have any ideas feel free to suggest and/or make your own variations. 
 

Pictured are printed in PETG (white) and matte PLA (grey). I've used both draft (0.24) and extra draft (0.28) settings and get good results with both, so I kept the 0.28 as the default in the .3mf.