UPDATE 2/1/16 - 7/25/2017
After a week or 2 of using the new 3d printed carriage I move the other 2 to 3d printed carriages as well.
I'm happy to rapport that after over a year of ~ 1350 hours of printing the 3d printed carriages have printed very well.
So one of my carriages for my MR12 rails started to bind up on me, doing some research on commercial linear rail glide options, I discoverer http://www.igus.com/
They have a DryLin® T - profile rail guides (http://www.igus.com/wpck/3555/drylin_t), line that looked perfect for my application. thou with quite the price tag.
looking over some options, I found iglide® tribo-filament for 3D printers.
using this i was able to create an fulley 3d pirnted adjustable clearance rail carriage.
you will need print out of iglide
I found that printing at around 230c hotedn and 80c bedtemp 20mm/s works best. Also print one at a time! helps to press some kapton tape down to help sticking. its a pane to get just right, but once you do you will get some very good slides!.
for the Carage.stl, SlideLock.stl, DeltaCarage.stl; print at 2mm 30% infill, using PET+ (http://shop.madesolid.com/pages/pet-filament)
some hardware is need needed.
6 Socket Head Cap Screw M3 Thread, 6mm Length
4 Flat-Head Socket Cap Screw, M3 Thread, 4mm Long
I sourced my iglide® tribo-filament form Amazon.
the DeltaCarage.stl is just the Carage.stl with added delta arm mounts for kolossal pro (40mm from edge to edge), belt lock,and end stop screw mount howl.