So, After some feedback I revised the spool holder slightly. It now features a thread end to secure it to the stock Wanhao i3 Plus spool mount. Also added a small lip on the end nearest the mount to prevent the spool from dripping between the holder and the mount. Suitable for small reels of filament although due to the size there may still be some slight wobble. I am working on another version for larger reel centers.
If you're having issues with the nut, increase it's size for printing to 103%. This fixes the tight nut issue :)
Wanhao i3 Duplicator Plus
File contains the spool holder and the securing nut. I would recommend you spin the nut on and off a couple of times before mounting it. It's a snug fit into the holder too.