How to display vertical MAME games on Pi2SCART without rotation?

[A Former User]

Ok, so I've asked this question before in the SCART thread, but had no luck with replies. I'm getting a bit desperate so here it goes again.

I'd like to know if it's possible to display vertical MAME games on my Pi2SCART + CRT TV setup, but without having to turn my 50kg TV on the "length" side every time. This means that there would be black bars on left and right but that's okay.

My current settings (240p) handle all the other platforms and horizontal MAME games just fine. Naturally, using these settings causes the vertical ones (>240) to be either squashed and ugly, or stretched outside the screen, losing big chunks on top and bottom.
I've been advised to use a 480i mode on the Pi (so the vert games would "fit") but can not find the proper hdmi_timings for that anywhere and am reluctant to experiment (already lost one TV doing that). Was wondering if anybody knows of a solution?

I'm using Rpi3B + Retropie 4.2 + Pi2SCART + 29" Sony Trinitron. My current settings are the usual ones from the 1600px fix found on RetroTink, Arcade Forge and other sites.