This is a simple spool holder for 3d printing filament. I designed it specifically for the smaller spools, but I have another instructable design specifically for the larger diameter spools.
To make this, you need common K'Nex peices and four bearings that fit over the rods. I used 608 bearings, which are used in skateboards and 3d printers. The spacers that come with the K'Nex may work, but I haven't tried.
Step 1: Assembly
Put four red rods vertically through a white fastener and push them all the way to one end. In the remaining four slots, put four yellow rods in the regular way, each with a bearing. Make four pairs with the purple semi circles or blue connectors. These act as feet for it so it doesn't roll around. Put these on the yellow ends and use red rods to connect as shown. The older style spools with the small hole 1.25 inches in diameter fit well and turn smoothly. The spools that have a larger inner and outer diameter and are usually black will slide around on this instead of turning smoothly. Check out my other K'Nex spool holder instructable if you have that type.