Below is our solution for mounting the rack solid. As Bob stated it takes some strange geometry to mount solid. The rack mounting surface is cylindrical on the drivers side and square on the passenger. The two aluminum clamp halves are machined to match the rack perfectly. Since we are not using the stock isolation material we had two choices for fitment to the horns.
1) Add material to the frame horns and shape to the rack mounting surfaces. The horn mounting surfaces needed to be built out after torquing the bolts.
2) Add material to the horns in the transverse direction to make them wider so the horns contact the rack in the transverse direction. You can see the difference between the clamp thickness and the horn thickness in the picture. The drivers side requires both sides of the horn have material added. The passenger side only requires the outboard surface.
We used neoprene strips to solve number 1. We purchased 1/8" and 1/16" neoprene strip and added it where needed to take make the horns tightly fit the rack shapes. It could be argued it is not solid mount. We'll run it and see how it does. It will be very rigid and full solid in the transverse direction.
To solve number two we will weld thin plate to the sides of the horns and fit the thickness required by the rack.
