Fabricating rear panels over wheel tub

[DRJDVM's '69]

So I have a set of stock Cuda rear interior panels that I’ve cut to fit the tubs and now I need to add to them to completely cover the tubs. Suggestions on material? I’ve considered shaping some foam and then fiberglass… best way to attach the fiberglass or other material to the plastic?

Is the a moldable plastic? Something that I can shape with some heat? Something else that's moldable to at least act a backing to lay some fiberglass or other material to foam/leather over?

The rear seat will go up against the tub but it wont cover the rest of the "arch" to the tub, so I want to extent that panel out to cover that section. The plastic panel has a bit of curve to it as it goes towards the package tray....slightly concave so there will need to be a curve transition from plastic to new section