I’ve been trying to do this for months, but the best I can do is create a screen output which has an aspect ratio similar to one eye. I think the ratio is 3:4, so you’ll want to create a window with that aspect ratio. Note that you’re not going to get “full screen” with this aspect ratio because monitors are usually 16:9 or 4:3. If you try to force it into the monitor aspect ratio, you’ll get the stretching distortion.
I’m just a noob at VR output though, so I’m sure one of the VR devs at epic has a real solution. I mean, they’ve gotta have one, right? If they go to shows with booths showing output on a full screen monitor, they’re doing something to do that…