If you want to go simple but like a different look, use the Vans quadrant and modify it. Paint the mixture knob red and the prop knob blue. You can also engrave M on the mixture knob and fill with white paint, P on the prop knob with same fill. Looks great. I added placards between the gates (black with white text) in the normal military format for the quadrant, and put a red placard on the side for Idle Mixture Cutoff.
You can also change out the throttle knob to something larger, or even put a PTT button in it with a little effort. While "store" bought options are limited, as was pointed out above; you are building it, so have fun and modify to your own needs. The Basic Vans quadrant works...but everything can be improved upon.
I like the vernier control idea a lot actually, but having said that, its really not all that hard to be pretty precise even with the vans mixture lever. Been using it for 600 hours and can pretty much set it with precision. Just go slow. Since I have placards (Throttle, Prop, Mixture) on the top of the three gates, its pretty easy to calibrate the mixture. For instance, I know I need to start leaning around the letters XT in the word MIXTURE and usually end up between the letters R and E. Using my engine monitor to find tune of course, but I can get close just by visual reference. As precise as a vernier? Probably not. But I can literally change temps by a few degrees. So close enough for me.