I wanted a nice looking filament spool holder to go with my Kossel Mini. I used the same 15x15mm OpenBeam or Misumi HFS3-1515 that is used in the Kossel Mini frame.
The lengths of extruded aluminum used was 3 x 15cm and 2 x 20cm so about 85cm of extruded aluminum was used. This should accommodate just about any size of spool. You can of course make it whatever size you want.
The rod spacer doubles as a means to prevent the rod from accidently slipping from side to side. It is made for something like 8mm or 5/16′ smooth rod. I made it to size but after slicing my printer made it a tad loose so I scaled it at 0.93% to be perfectly snug on an 8mm rod. You can scale the spacer to be snug on whatever size rod you want.