I assume the 12v positive source is from ignition right? That way your warning indicator isn't running all the time, but only when you are getting ready to 'take off'.
Soundguy.
Incidentally, that is what i finally did.. used the magnetic switches and a blinking red 12v led. I toyed with a feeler switch, but couldn't make it stay adjusted due to break wear, and spring tension, etc. The magnet idea is much better.
I put my magnet on the break pedal upright, and the switch on a bracket, very similar to yours, except that I didn't find a nice big magnet like you have, I had to use two smaller ones.
Nice job!