I had this need some time ago, and my nifty 5 digit Ebay panel meters had no provision for reading negative voltages. The solution was to connect the panel meter "ground" (measurement and signal ground) to the negative rail of the target bench power supply I was monitoring. The panel meter signal input was then connected to the target's ground rail, and the reading measured the negative rail's voltage, albeit without a minus sign. If the panel meter needs, say, a 5 volt supply, and the target's negative rail is not -5V, a small voltage regulator (analog or buck), can similarly be connected to supply the panel meter's need for +5V above the target's negative rail. Select a regulator capable of using the target's neg rail voltage, and sufficient current to supply the power meter. Hope this helps! / tomt