Designed these a while ago and have been using them for months, they're great.
There are two designed, one a big longer with some indents to make them look different, and a smaller/smoother one.
There are STL's for both designs in all variants (2 ends 3 bearings, 3 ends 4 bearings, 4 end 5 bearings, and 5 end 6 bearings).
These designs can be accessed here using Onshape if you would like to make any changes:
These are all designed for standard size skateboard bearings - 608ZZ - The universal measurements for these bearings are 8mm (core/inner diameter), 22mm (outer diameter), and 7mm (width). These can be bought very cheaply online, or from your local bearing provider, 3D Printer maintenance place, etc. After experimenting with some spare 608ZZ's I had from a 3D Printer build, I bought some decent "Cheapshots" skateboard bearings from my local Surf/Skate shop.
The holes for bearings are designed to pressure fit. I have printed them on my Cocoon Create (Wanhao i3 duplicator) and my custom built "Dwarf" printer similar to a Trinus. Due to backlash on printers, I have set specific tolerances to suit my printers. If your bearings don't pressure fit with these tolerances, feel free to open up the file in Onshape and change the inner circle dimension in "Sketch 1". Altering this dimension by 0.1mm increments will eventually find the size that suits for your printer.