Ammeter to voltmeter conversion

There are still plenty of those Sunpro voltage gauges floating around. There are five Ebay sellers offering them right now. I got mine from this place, good price, fast and free shipping:
Sunpro 2" Voltmeter 12V Black / Black Bezel New CP8215 Authorized Distributor | eBay

Sunpro was acquired by Bosch several years ago, but it looks as if Bosch is offering the same or similar gauge still under the 8215 part number. It's available for less than the price shown at many resellers.
Electrical Voltmeter Automotive Gauge | Style Line 2”