Yes, I've read a few threads and by everyone's opinion, the double wall is the strongest place to mount them. I also will run an aluminum strip on each side to use instead of washers. If the thickness is good, I'll use T-nuts and paint the strips and nuts white so nothing will be barely seen on the inside.
I got the right rail installed last night. I used the aluminum strip with 6 t-nuts and painted it all white. I'll cut off the extra threads when I take the hardtop off in a few months. The strip is the same length as the rail. Both 6.5" from each end of the top. Rails are 36 3/4" long measured without end-caps and are 48 1/2" center of rails side to side. Rail cut at the front end, new hole drilled there, then 2 more equally spaced between existing holes. All holes in rail, strip, and top are diameter of t-nut shaft diameter.
I first drilled rear hole into the top from the inside, drilled hole in the strip, mounted rail, then drilled hole for the front hole thru the rail into the top and to mark strip.
Then I removed all and drilled the hole at the front of the strip.
I then remounted all with rails and strips, front and rear screws and nuts, then drilled 4 remaining holes thru rails, roof and marked strips.
I removed all to finish all holes and paint and reinstalled using silicone to fill holes before screw install.
The t-nuts fit the screws that come with the fiberglass install hardware. I hammered the t-nut teeth flush before install, but they still have a little bite left to bite into the strip.
