Project Soli's tiny radar module is only 8mm x 10mm, and that includes the antenna array.
It's more precise than any other 3D hand-gesture sensor.
More information is on
https://atap.google.com/soli/Maybe this is a little premature for the "Making Stuff Together" forum, because it's not available yet.
But if radar can see through the key caps, put the radar array under the key caps near the index-finger key.
That way the user doesn't have to reach for a mouse.