The original Mk2 knob with the "O" ring works fine. Most any hardware store sells these for a few cents.

As for the knob replacement, it is actually very easy to solder the few surface mount contact legs. I had wanted to countersink my Mk2 knob a little (so I couldn't see the LEDs from the side) by lopping off the top of the knob. Unfortunately, the moderate heat generated in my cutting off the end trashed an inner plastic slip ring that holds the aluminum stem. I had to replace the original knob (only non-keyed were available), which was very simple to install. Had used my Mk2 knob for about a year with the O ring, through some very hot California summers, without any problem.