Well it doesn't look bad at all

Maybe simple straight lines would work just as well?
Unless you allow for semi-transparency, I don't think you can get anything better.
Corners: in the mockup view without rounded corners, it's just a matter of width: should be an odd n° of pixels. Just remove 1 px in height and in width, maybe a few stray pixels, and things should look good.
Same thing for rounded corners, plus some experimenting as to the best sizes for the circle arcs.
There are other issues in that
mockup screenshot though. Next thread!