Here's a different approach altogether. My Passport Escort 9500ix radar detector is mounted on my farkle shelf and displays the voltage as an option. Not that seeing the voltage is really necessary for me...
I think all of the somewhat current Escort radar detectors do that. I have a 9500ix and a Redline, both of them can display the voltage. Like you I'm more worried about staying out of jail, then the voltage.
I believe the 9500ix can be had relatively cheap now of days. Very good detector, prefer it over the Redline.