Do the eccentric spacers fit into the cube corner? I see several places where wheels are attached directly to the cube corner but I can't see if they are also using an eccentric. I've found the hole on the cube spacer to be 7mm and the diameter of the eccentric spacer to be 7.11mm according to the STEP files so it appears they won't fit together.
T They do indeed fit, export from Sketchup to STP may have introduced inaccuracy. You do need eccentrics on both sides
3 sides have the larger hole for eccentrics while the 3 sides opposite have the smaller, standard, non-eccentric hole. Most eccentric spacers fit easily but the occasional spacer may require convincing.