I have tried holding to the spool edge and having it slip from my fingers and unwind the string just to start over again. This holds on to the spool by way of an 8mm hex bolt and the seven catches of the spool itself. I had a 20mm long bolt but a 10 or 15 will work. The pins catch the spool preventing it from turning while the bolt keeps it all together. Just finger snug the bolt to hold it together. Tightening it down could break the layers of the spool holder. Since the cap is not a press fit for the hex, it will need to be glued in. I used Gorilla Clear Grip Contact Adhesive. It fills in the gaps between the hex and cap. I used 3D Fuel LulzBot Green PLA and generic black, 0.2 layer 3 walls, 25% infill