looking for advice on the undeside of the trunk lid on what material to use

You seriously need to be way more specific in your question - both the text description part and pictures of what you're trying to achieve.

What are you talking about exactly ??
the small lid at the bottom of WRC bumpers for example ?
If yes that can be made with paper covered by carbon fibre decal for example...

I'm guessing the reference is to NASCAR Nextel Cup cars. The COT cars are fabricated in a similar way to the older style cars, the trunk lids are sheet steel or aluminium with a crossbrace of tube steel to support them. If you look at Dave Thibodeau or Clay Kemps builds, they replicate the crossbrace with plastic rod. I'd suggest 1mm rectangular rod cut to to match the tubing in your reference pictures.