What about mounting it sideways?

Can this be done with foil or metal?

Based on a layman's knowledge of these things, I'd guess that's probably a bad idea there since the microwave reflectance off the metal could saturate the receiver on the sensor. Best case, the hot signal confuses the sensor making it unreliable for tracking objects at a distance. Worst case, it could shorten the device lifetime if not outright burn out the receiver.

Rather, you'd want something to absorb the microwave energy like, say, paint with carbon black in it. It'll still covert the absorbed energy to heat (like your microwave oven), but at the power levels we're discussing you could at least dissipate it somehow.

Edit: just realized I didn't address shaping. What I mentioned above, I was thinking of sticking a strip of foil across a portion of the plastic lens. You could probably form a sort foil visor for the sensor, like a ball cap, but you may still run into issue with false positives and ghosting of objects as signals would now be bouncing around in ways the sensor wasn't designed to account for, if they even get picked up.