Well I think I got it figgered out. Turns out the Qparts knobs already come with a sleeve inside them, which you can remove if you need to. I took the sleeve out and tried the knobs again and they were way too wobbly, so I put the sleeve back in and I finessed it so it was level. I had to hold it really carefully with one hand, and only tighten the set screw the minimal amount. The more I tightened, the more it tends to tilt to one side or another, so the trick was to tighten it minimally. Now they seem level. They work fine and they look great. Oddly, thanks for tipping me off to these knobs, and Kev1955 thanks for the info about sleeves -- I wouldn't have even bothered to look if you hadn't mentioned that.