Lots of digital panel meters are available at Amazon, etc. One of those plus a switch would work (though without the needle, of course).
Here is a nice looking analog gauge for battery voltage, with an appropriately sensitive range. I don't think it would be great for your purposes, though, in as much as any converter would peg it on the high side. When operating off of battery power it would be fine; maybe have a momentary pushbutton switch to connect it in under those circumstances. (No numbers on the scale, unfortunately.)