I was wondering if it was possible to dissolve the PU 80 low density model board in some solvents after having laminated a CF cloth around it ?

You are on the right lines with the wooden core, but I would model your part in wood, or machine it from aluminium or dense tooling material. Then I would make a female mould from it. For a phone case I would just make a top and bottom half and then join the two sections.