Hi,
First congratulation for your rear tube: it looks excellent !
Second, as you say, some GRP panels are straight, as mine are. It does not interfere with ther lower wishbone, as the pivot point is 25mm (bottom rail) + 30mm (bracket) = 55mm higher than the lower face of the bottom rail, so it allows a bit of downward movement to the wishbone.
2014-08-07_15-42-02 by
Sylvain ROIG, sur Flickr
2014-08-07_16-36-22 by
Sylvain ROIG, sur Flickr
The main question for you, is how are you going to attach the front of your rear panel to the bottom rail ? That is also the reason why there is a fold in the book's ally side panels. My GRP panels have a 90° return underneath the panel.
SidePanel-NoDims by
Sylvain ROIG, sur Flickr