I stopped by my local hardware store and purchased a couple of spools meant for electrical wiring. Trying to roll the 5lb roll of filament onto it was a COMPLETE disaster... to cut an embarassing story short, I now have many sub-1/4lb rolls of ABS instead of one 5lb roll.
So instead, I removed the handle from the spool forcibly, and cut the axle in two parts (but not half! See photos!) so that I could take it apart and fit it through a roll of ABS, hopefully to greater success.
This pin can be used to join the two pieces of the spool into one again once the plastic is on the spool.
The OpenSCAD file for this is just plain awful. I'm including it out of open-ness, but it's not worth having. It is the first thing I've done in OpenSCAD (or at all really) so I wasn't trying to be proper, just playing with some basics.