[HO455]

Once I was satisfied with the lay out I cut the panel. Staying outside of the lines (just in case) I made the cuts using a jig saw with a metal cutting blade with the orbit setting at zero. After cutting I cleaned up the cuts with a long sanding block(with 80 grit) to remove any wandering of the free hand cuts. I also rounded the corners to prevent chipping and beveled each edge so it would slide easier. Before trying to install the panel I took a small screw driver and went made sure the inside of the channel the panels sit in was clean. It took several attempts to get the panel in place. Each time I had to make the panel smaller. Basically going 1/8" at a time getting the panel to fit correctly. You don't want to get it too narrow or it will be held up out of the side channels by the plastic support trim. As the side channels on my Burban end at different points from the rear hatch header I had to just get it so it looked good across the header and ignore the bit of FRP hanging past the end of the channel on the one side. Once I got it to look reasonable across the back, I then concentrated on getting the front edge to match the dimension of the support bow.
By now I have installed and removed the panel at least eight times. When I needed to reduce the size in small amounts (1/8" or less) I found that sanding the material away worked better than trying to cut it. As I had the rear of the panel to size each cut was a long wedge. When installing the panel you have to let the center droop while you put the edges into the channel then push the center up until it pops in place. If the size is too large the center won't stay up so you need help to hold it up while you measure and mark. I didn't have an extra human around so I used the piece of PVC you see in the picture to hold it. (Photo 1) ( It was about 8' long and I recommend covering it with masking tape it where it contacts the FRP.)
Before installing the panel for the last time mark the center line of the roof stiffeners. I did three screws on each stiffener. (You know, more is better!) I only marked the ends and didn't think to check the center and it came back and bit me. On the rear most stiffener it was bent and the center was at least a 1/4" closer to the back than the ends were. Which in turn caused me to miss the stiffener when I drilled it for the screw. Of course none of the rest were bent but I had measurements for all the rest.
Once I felt confident about the size I carefully fitted the support trim in place and snapped it down. I started at on end but that turned out to be a mistake as the center must be in place so the support trim will push the FRP up and follow the bow. Photo #2 shows what happens when you don't start in the center. Once the trim is snapped down I was unable to get the FRP to slide side to side and so I had to carefully unsnap the trim. Fortunately it has been in the 88 to 93 degree temperatures here so that helped as I just had to set the pieces out on the asphalt for five minutes to heat them up. The down side was using the heat gun inside the Burban to help with the support trim. Hot hot hot!
Just for clarification I drilled and screwed the panel after the support trim was installed.