This is my take on a linear bearing, there are better out there. I just wanted to learn polar repetition in #FreeCad and have a bearing that printed under my sloppy tolerances.
So the InnerDiameter in freecad ar 4.1mm radius, which once stuffed through a cura2 slicer produce a nice fit that needs a bit of dry lubricant (for my sloppy tolerance). The ribs would be improved with a helicular pattern, but I haven't learnt that yet.
I have provided stls of lm8uu (24mm) lm8lu (45mm).
The freecad files are attached for people to mess with.
I use a 0.3mm nozzle, so my shells are multiples of that.
I have found 9mm and 12mm shells give the same pattern in cura, yeah there's gaps. But I bet you can't nand crush or snap the output when printed at the right temp and speed. Speed and temp are your choice based on material.
I used PET-G. I might go back to PLA and see how they slide.
Try to avoid an inner brim. Outer brims are fine if you need them