A reel stand to mount on top of the frame. A 5/16" bolt or shaft in the gap will act as an axle. I made this after seeing several other designs on here. Didn't see this design, but I would not be surprised if one showed up. I wanted to decrease the footprint of my printer. I will say this is not the "best" design. When the extruder is nearing the top of the frame, and if the spool is too full, it can make a sharp bend in the filament. Potential to make the motors miss a step and lose alignment. Also, I think I should overhaul it again to reduce the amount of plastic used. It does not need the trusses on the frame; it is plenty strong as is.