The plastic plate can be any type of plastic or synthetic piece. I used this really light plastic that is kinda like foam.
Line up the mount in the middle of the plate and mark the holes with a permanent pen.
Drill holes in the plastic plate that line up with the speaker mount... mounting holes.
Using a pop riveter attach the plastic plate to the speaker mount. You might have to use a small washer to give the rivet a good foundation to grip into. When you do the pop rivet you have to go from the bottom up... otherwise you might run the rivet into the projector later.