You can solve this by doing the scaling on the GPU and not the display.
With Nvidia cards this is under "Adjust desktop size and position", select "Rift DK1" then under "Scaling" choose "Aspect Ratio, then "Perform scaling on" -> "GPU". AMD should have similar options.
This will fix the aspect ratio skewing issue. Otherwise you need to feed in a 1920x1200 or 1280x800 image.
Thanks cybereality, that does indeed work. I didn't know that was an option in my drivers, guess that's my "learned something new" for the day.
I'm having the same problem with AMD RX480.
The problem is that the amd drivers don't seem to see rift as a display. It only shows my primary monitor which is 1600p. I can find the scaling option for my monitor, but no rift there.
Anyone know a solution?