The original Ultimaker caps add useless friction to the X/Y axis bearings, so I designed these instead. Best printed in Nylon for lower friction and more durability (or check the many derivatives of this thing!)
Update: added rounded/smoother version, slightly bigger and with optional bearing access hole (eg. to oil it if ever...)
You need 8 of them for a complete set. Some of the caps of an Ultimaker are hard to change. The easiest way was to use curved long arm pliers to hold the nuts, before I made a convenient "nut calumet" here: http://www.thingiverse.com/thing:44118
These caps are fully parametric and were made with openscad, so they can be recycled for another use (ie. mostly keep any bearing in place on a flat surface). If you want your axis to go trough, ask me, or add a difference() with a cylinder centered at 0,0.
When printing, you can keep the fill ratio quite low, because there is no load on these pieces: they are just meant to hold the axis bearing in place. Anyhow, do not expect a big saving because the piece is small and mostly made of walls (~20cm of 3mm filament).
There is a small (parametric) conic pin in the middle, made to avoid too much lateral freeplay for the axis. I suspect it is useless though, and would wear out quickly anyway in case an axis really want to escape sideways ;)
About the stock friction issue: http://wiki.ultimaker.com/Ultimaker_rev.3_assembly:_X-Y_axes#Step_1:_Inserting_the_ball_bearings
I forgot to take pictures of used wooden caps, so I borrowed one from maxy, thanks!