so they don't roll around and are easier to grab

made for the "new" cylindrical tubes, but the old rectangular ones can fit too, but won't be held up (but you probably have more cylindrical ones, so it will be held between 2) ... but but but but

on the ultimaker², for 72 tubes, it takes less than a day to print 15 rails and 16 assemblers

for different printers, do just 1 rail and 2 assembler to see if they fit and doesn't have too much play (you have to be able to put 2 assembler next to each other in 1 hole of the rail,)
if you need to change the adjutment, only modify the rails
the 2 only remove feature, first one where you will have to change the diameter of the mounting holes, and the last one, the height of the grooves