Get something that is shaped different would help too!

I’m not sure this is the correct size (6mm)
http://www.gravitech.us/blrukn146mmd.html

Or if shape doesn’t matter you could get the knob upgrade from Elektron.
http://www.elektron.se/content/mdmono-knob-upgrade-kit

Here’s another:
http://www.mcmelectronics.com/product/28-11826&scode=GS401&CAWELAID=1292681692

You just need to know the dimensions of the length and width of the shafts. Search “d-shaft knob”