Parametric Enclosed Stackable Spool Holder

Update 5/5/18: I am satisfied enough with the design and the fit that I consider this to be finished, which isn't to say I won't keep it updated if there are errors found or someone has a good suggestion.
Update 5/9/18: I have recently designed another style of enclosed spool holder that doesn't use a central spool core - https://www.thingiverse.com/thing:2900930

While inspired by the Spannerhands spool system, I wanted something a little different so I designed my own enclosed spool holder