Gaps between GRP panels and chassis rails

[voucht]

Thank you for the reply Jason, but I really don't see how the GRP can take the shape of the tube: the 90° corner of the top edge and the side panel will stop me to achieve that I think. Look at the first picture of my post : I think the surface to shape is way too narrow for that (25mm wide), and the gaps are too big.
But thanks for the tip