you could get a hole saw and cut a whole in the plexi big enough for the knob to stick partially through. Or, my prefered option, de-solder the pot from the board very carefully and use the nut and probably lock washer to secure it to the plexi, using wires to attatch it back to the board. Also get some panel mount audio jacks, or extend the existing jacks and hot glue them to the plexi from the back.