I made the threads with this http://www.thingiverse.com/thing:193647 . I like to keep these hubs on my spools for easy switching of filaments and it fits most of the spools i use including the cardboard ones. For very narrow spools you may need to trim some threads off by slicing a portion off the top of the bolt portion using 3d builder or other software or just by stopping the
print. I like the look of the bearings whether needed or not , and I usually buy extras or salvage them from roller blades
.printed with .24mm layer height, 15% infill, with or without support . If you print in ABS use support, as it doesn't bridge as well as PLA
You may use a drill or reamer to clear for a 8mm or 5/16 pin, rod or dowel. Snap in 2 608 bearings for a free wheeling spool or don't clear out the inner diameter so there is some resistance.