Lots of good Lego bricks models already out there, but I couldn't find any with holes for axles so I made my own. This is a fully parametric OpenSCAD model with all dimensions easily tweakable.
Nominal values for each dimension are listed in the comments, and the values shown are what worked well for me after painstakingly tweaking each one for my Prusa i3 MK3. If you want to use this model, chances are you'll need to tweak a lot of the dimensions as well to get them just right. The .STL files attached include tweaks for my printer, YMMV.
The axle holes aren't perfectly round since the tops of them form a small bridge. I've been reaming them out with a 0.191" drill bit and then they work great.
Using a smaller nozzle should help get the dimensions right, smallest I have is 0.4mm.