OK...here is a part that I am making. Here is the mould. I did it with the EC gelcoat and paste for high temperature, anticipating that I could then use it for hand layup and prepreg. I am making my first part by hand layup (because I need to check all dimensions are good at the end). But man it was a mess with all the shapes...do you guys think that I would really be better with prepreg or would it be just as hard ?
Thanks !

No, that won't work. And if you might get it down the part will look horrible, because the fibres will be disoriented quite a lot. The main problem is the prismatic shape in the middle. Fibres which go across this shape will need to be much longer than fibres which run flat along the shape. This will leave a heavily diagonally streched area on both sides of the shape. This isn't a prepreg problem, this will always be the case.

Thanks ! Now if I don't mind something that is not good aesthetically speaking, is it doable ? If not, do you think that I can use a single piece for the entire part except for that prismatic shape in the middle, cutting my prepreg just at the edge of it, and then use separate parts for that prismatic form ?