Thanks everyone, my initial idea was to use VHB, but I was a bit nervous and was thinking "belt and suspenders" by using both VHB and Riv Nuts. I've been reading the spec sheets and watching some videos, so VHB it is.
My situation is a bit complicated since my top has a slightly crowned ridge down the middle. It's maybe a foot wide and my panel is about 20" wide. I think I have a plan, using VHB and aluminum angle stock.
1. A couple of small brackets on the top of the crown, front and rear (two each so the slight side-to-side curve won't be a problem).
2. I'll then add some triangular brackets VHB'd to the sides of the crown, front and back, extending upward to the front edges of the panel. These will support the cantilevered parts, and may make the center brackets unnecessary, so I'll do these first.
3. Since I've already purchased the Riv Nut tool and nuts, I'll use them to secure the panel to the brackets.
This is probably overkill but I do tend to over-engineer things.