This is a tower spool holder for the RepRap Mendel. It is designed to not twist the top threaded rods, and to supply the filament above the print area. Its height and thickness can be customized by editing the scad file. I use a 125mm threaded rod with nuts and fender washers to secure the filament spool in place. I find that the threaded rod provides just enough friction such as to prevent the spool from unraveling. Secure it to the top two threaded rods of the machine with a nut and washer on either side of each hole.
Update 31 DEC 18: Added tolerance factor to scad to allow for properly printed hole sizes.
Folger Tech RepRap 2020 i3 (expanded Z)
Printed with 0.6mm nozzle, 0.75mm extrusion width. Use max 8mm^3/s volumetric speed. Use a 5mm brim. Use 230C hot end, 105C bed temperatures. I used a Folger Tech RepRap 2020 i3 upgraded to have a max 310mm Z height, and printed vertically. If necessary, change the height in the openscad file and regenerate the stl to fit your machine or desired layer orientation. The HIPS I'm using has very good strength and layer adhesion, but you may have to print horizontally to get better strength with other materials.