This is an above bot spool holder for Steve Grabers' excellently designed Cerberus Delta printer found here: http://grabercars.github.io/CerberusPrint with enough infill to make yourself comfortable (50% and above should be good enough). Slide on to the top rails at the distance you want, use a rod to hold the spool (I use 1/4" threaded rod so I can put nuts on the end).