Depending on your manufacturing process, the mould doesn't have to be rigid at all. I've made an angled plate by applying two 2 mm steel plates to the original chassis with pieces of tacky tape (like chewing gum) put into the counter-sunk holes. The lay-up was CF prepreg twill and UD and the vacuum bag held everything together.
If I was really serious about making a good CF chassis plate for an electric car, I'd use two angled steel plates for moulds and a sandwich lay-up with CF prepreg face sheets.