Thanks! Turns out I'd have to put the shanks almost at the very top, even with a 12" collar, to allow the drip tray to be flush with the base. I decided to put the drip tray part of the project off until another weekend, and build a descending attachment so that the shanks can be about 1/3 down from the top.

(drilling in progress - easily the hardest part of the whole project)