If you would like to mask the cutting line in order to not see the core, you can use CNC cut strips (0.5-1 mm thick and lets say 8 mm width) of carbon fiber sheets or U profiles and glue them to mask the core (see attachment).
In order to make a box you can use cut and hold method, it means...
There are many materials that can be used as a tooling material.
If the mold is not split type that needs to be screwed together you can use soft material - epoxy tooling board - for instance Renshape BM5060. This material is temp resistance up to 140 C and very easy to mill / machine.
If mold...
Parts with complex geometry, especially with many curvatures and splitted molds generally needs autoclave curing (high pressure).
High aesthetic parts where you need carbon fiber look at top-level of quality needs autoclave curing as well.
Heated molds with vacuum bag generally speaking will not...
