Rear bin for K2 Plus

Uses existing (short) screws for the spool holder on the K2 Plus. The screws don't have a lot of extra length so it may be best to replace them with longer ones, but it will work well enough with the stock screws.Adds depth to the printer, so this may not be what everyone wants. I designed it for a place to keep all the filament clips since my printer sits sideways on a cabinet and I have access to this side and rear.