Hi,
My suggestion thinking this through....
Find the center of the hardtop at the mounting point as stated.
Pencil the NS-EW lines on the hard top and center rigger mount itself. Position and locate the hole centers on the hard top. As always - measure twice - drill once....
Determine the size of your bolts to be used for the rigger mounting points. The lenght should be sufficient for the entire stack up (the rigger thickness, the hardtop thinkness, the backplate thickness and finally just enough for the washers and locknuts with a thread or two to spare).
I'd use SS bolts, nylock washers, lock washers and nylon flat washers. Use the latter between the heads of the bolts and the center rig mount and the same on the bottom to isolate metal surfaces. Get the tube spacer material. This is basically a hard plastic hose that is the next size up from the bolt diameter (the bolt should fit fairly snug inside the tube spacer material). Drill the holes in the hardtop to the diameter of the tube spacer (it will be larger than the bolt size). Cut the tube spacers to the hardtop thickness. These basically are a sleeve for the mounting bolt set in the hard top. Test fit the spacers to ensure they do not protrude above the top and bottom of the hardtop plane. Spread 5500 on the tube spacer outer body and press into the drilled hole. Repeat for all mounting locations. Make sure there is a nice seal top and bottom between the tube spacer and the hardtop to prevent water intrusion. The tube spacer is used to ensure you don't collapse the hardtop material around the bolts and also gives you a water barrier. Place 5500 on the bottom side of the rigger and set in place, drop the bolts through the rigger and hardtop. Install the backing plate (with 5500 if flat and you can) and finally the stacked hardware. Snug the first bolt, then the opposite diagonal bolt then the other two. Tighten snug in a circular pattern. Don't over tighten. Wipe any excess 5500 around the perimeter of the rigger top and bottom. Once the 5500 sets you should be good to go.