After the fiberglass board has cured you can then proceed to drill your backing plates. This is easier said than done! You will need at least a 5.5 to 6.5 inch hole saw and a very large and powerful drill.
The friction of the six inch hole saw, on the fiberglass hole, is tremendous and can and will kill a wimpy drill. I use my drill press for this but a large and powerful 1/2 inch drill from the likes of Milwaukee, Porter Cable or DeWalt is a must. Home Depot will rent you large drills and I advise this if you don't have a drill that is up to the task.
Trust me you will kill a Black and Decker or Ryobi before you even make it half way through your first hole!
One word of advice on drilling these backing plates is "CLAMPS". By this I mean always clamp your work down before you begin drilling or you'll have a giant spinning fiberglass saw on your hands.
Circular fiberglass backing plates are certainly the best looking but tough on the hole saws and too much for cheap drill motors. Another practical alternative is to cut a hexagon or octagon with a miter saw or table saw. This method will be faster and easier but just not as neat.