As far as the construction is concerned, you'd want a wooden frame/block all the way around the perimeter of the control panel to reinforce the plexi. I found that the thickness of plexi has very little bearing on the amount of flex. For instance 3/16" at 10" x 24" is solid enough to stand on with the framing I'm using (5/8" plywood). Without this framing, even 1/2" of plexi had a ton of flex to it.
And if you're worried about the flex of the plastic touching the canvas and messing it up, you might add some space between the 2 so that the plastic has room to move a bit.