Poster board, not cardboard
When I am shaping something complex like this I use poster board (I think its called Bristol board still in some places). Basically really thick paper stock. This lets you press it into place at the sharp bends and make cuts in place.
I’m also looking to purchase this side panel because I already have the matching leather stock from when I did my seats. And frankly this is the only molded panel I see available.
Just wondering how yours progressed.