Refer to this lengthy 6-page thread from last year:
https://ogrforum.ogaugerr.com/...digital-pannel-meter
The thread meanders through all the various issues (such as RMS vs. not-RMS) so it can be hard to follow. I don't think there's a Reader's Digest condensed version.
Given the lack of economical options, I demonstrated how to modify an inexpensive (under $10) eBay combo Volts and Amps meter to make it more suitable for O-gauge AC. But it is NOT a practical alternative given the tedious handi-work involved.
https://ogrforum.ogaugerr.com/...71#73368898730965771
The point was it would be trivial for a manufacturer to modify an existing AC combo (Volts and Amps) meter to better match O-gauge applications. As to whether they could sell enough of them is another matter.