Setting a custom PBP resolution using CRU

I have an Acer X34GS (native 3440x1440 resolution) which supports PBP, but doesn't have 1720x1440 as a supported resolution in it's EDID. I'm trying to add this resolution.

What I've tried:
I have a Nvidia card, and so I've added a custom resolution in Nvidia Control Panel. This gives me a Desktop Resolution of 1720x1440, but an Active Signal Resolution of 3440x1440.

This isn't a problem if I set scaling mode in Nvidia Control Panel to No Scaling, and then set my monitor's PBP mode to Full instead of Aspect Ratio - the 1720x1440 image stretches over to 3440x1440, and then the monitor forces it to half the screen, essentially unstretching it.

However, I would like to have an actual native resolution of 1720x1440, so that I can keep my monitor in Aspect Ratio mode (so I can use it with my Switch keeping it in 16:9, and also because the stretch-and-unstretch makes text look thicker).

FWIW, I can achieve this using SwitchResX on macOS, and so I know that a native 1720x1440 resolution does work.

Using CRU
I've tried adding a new resolution in the Detailed Resolutions part of CRU, but it doesn't appear in Nvidia Control Panel nor in Windows display settings. Any help would be appreciated!