I used a piece of ABS Plastic (purchased at Tap Plastics) for my mount backing. This plastic won't crack and provided strong support.
Make sure do the exhaust side bag first. Set the height and angle you want then drill the holes. I would actually mount this side first completely then do the other side. Hopefully you can have someone help you out letting you know if your even on both sides. So...
Install muffler side first, then use as reference for other side...
Leave plenty of room for passenger pegs to work comfortably!