Hi, I would like to make angles similar to the pre-preg ones on sale on the EC website, but in epoxy + fiberglass.
Thickness has to  be 5mm.
What is the best way to build the mould?

You could do it with an inner mould as well as a compression mould.  Clamp the moulds together with some wood as a load spreader and wind up the load.  That way when it's cured you will get a gloss finish on both sides of the angle. 

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant