The famous cubes-on-elastic-string puzzle, now customizable on your 3D printer. Either print the provided STL or customize it the way you fancy it. Printing is easy, no support or post processing required and the chamfer at the bottom will take care of any elephant-foot your printer may cause.
I tie them with 2.5mm elastic string (a few rubber strands enclosed in a cloth sleeve). Make sure to stretch the string before you tie it to have holding the pieces firmly.
The design have two kind of pieces, regular ones and end pieces. The end pieces have an extra cavity on one side to contain the knot.