Hi Kris,
I had no problem reducing the size of a bobbin.
I could not find the correct size dowel to use as a spindle (arbor) so I cut a 3 inch piece off a regular pencil. I tapered one end with a utility knife then locked the other end in the chuck of a power hand drill. I used a file to smooth out the tapered end so the centre of the bobbin would fit onto it snugly.
I pushed the plastic bobbin onto the spindle and held the coarse file of my Leatherman against it, the diameter was reduced quickly. I checked it a couple of times for fit and finished the job with the fine side of the file.
The bobbin now drops easily into the bottom of the matchcase. It now has a diameter of 0.750 inches and a capacity of 25 feet of 50 lb Spiderwire.
Wear safety glasses when doing this modification as the bobbin flew off the spindle once.
Neat idea Kris!
Mike