I like the square type styling of your current design and I agree with your simple style knob design. If you used one of the single shaft units you could use your black/chrome knob and just tilt the knob for the directional control.
There are several units available off the shelf that would work for a single shaft setup and would match your styling. That way you could keep cost down by not having to tool up to make a custom knob. Great work so far, I'll be on the list for one of these regardless of what knob you use.