Turbogoat324

Hello I have a 1988 rx-7 convertible and I recently purchased a shine diffuser for it. I am trying to install it and I understand how the side prices mount and the sides of the center piece then attach to those. Where I am confused is the back. When I hold up the diffuser it hovers over the gas tank and obviously I can't drill into that. Also the 2 brackets that came with the hardware bag only have 1 hole drilled for each side. Am I missing something. How do you bolt this darn thing up. HaHa. Any help would be greatly appreciated and pictures would be even better. Thanks!