Hub for Heavy Duty Filament Spool - parameterised

I'm using the excellent "Heavy duty printable filalment spool" http://www.thingiverse.com/thing:10219
and was using the elegantly designed "Ultimaker mount for Heavy Duty Spool" http://www.thingiverse.com/thing:33461
but wanted something more rugged.
There seems to be a general reluctance to drill holes in the frame of Ultimakers, but I have no such inhibitions which freed me to design this simple hub
There is not much room on the inside of the Ultimaker to accommodate nuts, but there i