There's prob. an easier way, but first thing that came into my head was get a metal rod, similar diameter to the one in the switching box you have now, cut an X groove into the metal of the switch/knob, and carve out a similar shape into the metal rod you purchased. (could even do a solo groove, but it would have to be secured better to prevent slipping).
Then just put the knob on the top of that extended piece. Would require that carved out too (would be detailed dremel work).
Just first thing that popped into my head.