In order to create a mask, I plugged the projector into a ceiling outlet and set it inside the crown assembly, and got the mirror calibrated. When I turned it on, and set up the mirror assembly, I taped a piece of paper underneath to trace out the mask with pencil.
Once I had the trace, I created the shape in Adobe Illustrator and laser-cut the mask out of 1/8" white acrylic.
I used a series of neodymium magnets so there would be no visible fasteners, and as a bonus I can remove the mask easily if I ever need to access the projector from the underside.
Also during this step, I created supports for the sculpture in the base. After a little trimming of the edges, I made it fit flush in the square recess.