How to "Optimize for" external monitor, but supersample for clear text, similar to leaving macbook open and saying optimize for built-in retina display "looks like 1440x900"??

In Display settings, it's driving me crazy that I can't leave my Macbook closed and supersample for scaling OSX and maintaining image quality. I currently have to leave the Macbook open, mirror displays, and say "optimize for" built-in retina display ("looks like 1440x900"), which looks great (larger and clear), but leaves black bars on sides of screen and Macbook has to stay open.

If I simply optimize for external display and change resolution, it's sampling OSX at the lower resolution, then scaling to fill actual screensize, so everything looks blurry and it's even harder to read text than if I just left it at high DPI, original resolution.

How can I supersample the same way, but with Macbook closed and different display aspect ratio (no black bars)?

Thanks