The upper half of the clip rests on the rim of the case around the bezel, with only the end of the foam pad against the glass:
That’s pretty much the only stable position.
Sticking a disk of stair-tread rubber on the foam adds just enough thickness to match the rim:
The lenses came with two clips, so I left one unmodified to fit the Pixel without the case:
Not that that happens very often, but …
The lenses are about as good as you’d expect for ten bucks from Amazon. Stacking the 0.67 “wide angle” lens on the camera enlarges the field-of-view by a third with closer focusing at maximum zoom, so the minimum FOV drops from 2 inches down to 1 inch at a reasonable distance. The 10x “macro” lens is basically useless, with a focus distance well within the Pixel’s shadow under any normal lighting; if I were that sort of guy, I’d conjure a small LED ring powered from the USB-C port.