Adapter for Filament Spools

###Update
I changed the cone from 22-28mm to 22-32mm, so it can be used with spools from CCTREE without falling through.

####Summary
Filament spools have various sizes of their inner holes. I've designed an adapter to cope with that.

The adapter uses a konus to adjust to individual spools. The adapter's inner hole fits onto an 8mm rod.

I've intentionally not used any ball bearings, because the spool must produce some resistence when unwinding the filament. My earlier trials with ball bearings didn't have such resistence and ended up with too much unwinding and clogging of filaments.

FreeCAD v0.16 source files included. Feel free to modiy to your needs.