If you have a fiberglass replacement 4 speed tunnel, you have a great template. Lay it in place, and use a marker very close around the outside of the glass tunnel, and set the glass piece aside. Determine how you are going to attach the glass to your floor, and add as much material as you will need for attachment onto the inside of the line on your floor and make another line. Cut the floor on the inside line. If you need to bend a flange to attach the glass to, you can cut a slot in the corners and bend each flange to what ever angle you need. Attach the fiberglass 4 speed tunnel to the extra material you left for attachment.