Mark a line across the back and cut it with the rotary tool.
Mark a line from your cut to the plastic on the sides and cut so the back piece of plastic which held the lid on comes off.
do the same for the front.
Remove the thin lip along both sides with a stanley knife. (This lip protrudes above the glass a fraction)
Clean off any burrs
Sit the glass in place and make sure that the glass is the highest part of the scanner.