Panasonic has some 30 and 35mm long shaft pots at digikey
EVU-F2LFL3B24 read the pdf for this one there are lots of options in the series.
I went to digikey then clicked on potentiometers, then hit the 20k resistance and filtered out all of those. It's the first on the list of many
They also have a filter for 30mm and 50mm!!
here's the link to a POT 20K OHM 2W WIREWOUND LINEAR with 50mm shaft
http://www.digikey.ca/product-detail/en/43C120K/43C120K-ND/1956260