UPDATE: I've found that when you orient the spool this way, slack in the filament can create loose loops of filament on the spool, which can be pulled off the side of the spool as the print head moves around, so be careful that this doesn't happen and jam your printer.
Just add velcro, and you're ready to go. Attach it to your printer, or put a row of them on a nearby wall. The peg is inclined 5 degrees to ensure the spool doesn't fall off.
This holder is known to fit Hatchbox spools and Solidoodle spools, and its overall dimensions are shown in the gallery. Units are inches.
This is a relatively large print, so make sure you have good bed adhesion. Slight warping is okay, especially if you're using velcro or foam tape to mount it.