In a production set-up from a fiberglass female mold you could pop them out from a sheet that has been laid-up. In one-off you have a tolerances difference from side to side - I find it easier to fit and trim the foam first.

Also it is easier and less itchy to trim soft carbon that has not fully cured with a razor knife on each piece then to saw up a hardened glass sheet and sand to fit. (Unless somebody else is sanding!)