For me, the disks are 3 5/8" (pictured) to 4" in diameter - that's what fits my hand so I can hold the ends of the bolt with thumb and forefinger and let out line. The one pictured holds 75' of braided #24. I have others with a bit longer bolt that hold about 120' of #24. If you have trouble controlling a spool like this (no finger holes, just bolt ends) you can put a couple of the tops off medicine bottles with the tops toward the middle under the bolt ends to make little finger pockets.
If Illinois is a "two plate state" (in Alabama we just have a rear license plate requirement, so all sorts of plates are available for the front - most made out of reasonably tough, flexible plastic) then you'll need to find a source of side material - I've also made 'em out of the sides of pool chemical buckets & stuff like that over the years.
Have fun...
Rick