I've done the Pitbull SR kit myself. The IB5. The build thread is here, in case my sig file and links aren't displayed on your device.
I made a rebate template, just like you have, but from half inch MDF. I then clamped it to the back of the body at a slight angle and it was flexible enough to follow the body curve to a degree. Luckily it worked well. The specific part about that is here.
Try a dry fit first. It might be flexible enough.