This spool roller is an evolution from the one of jstck (http://www.thingiverse.com/thing:307228). It is designed to fit on the "top" plate of the LOGresse P3Steel printer (https://www.logre.eu/wiki/Logresse).
There are a few supports to allow a good geometry of the slot (see picture with red line). These supports are very easy to remove with a thin plier. Then, depending on the quality of the bridges your printer made, it may need a slight finishing operation (with a thin file) to get a perfect fitting of the slots on the 3mm steel plate.
I mounted each part with two 608zz ball bearings, and two axes made of a 8mm diameter aluminium tube cut at a length of 17mm. You may have to redrill the holes with a 8mm drill to get a perfect diameter allowing the insertion of the aluminium tube (and then use a wooden hammer for a soft insertion). But another good idea would be to print and use the rivet specially designed by Gush (http://www.thingiverse.com/thing:1697239).
Insert each spool roller diagonally on the top of printer, then rotate to put it straight. Then you can adjust the distance to fit the width of your spool.
Two versions are proposed. The older one that had not been published is symmetrical shape, and fits for most of spools. But it appeared that it cannot be used for very small diameter spools that are sometimes used for special filaments (e.g. filaflex).
A second version (called "asym") is proposed to fit all diameters. It is a bit weaker than the previous version, but still strong enough I think.
The main photography shows both versions together in place on my printer (showing also that it can be used for a double extruder printer).