Have kind of worked out making the mould but laying the carbon fibre (or similar) is way out of my scope.

What you want to consider though, is will this thing become dangerous in itself in the event of a crash? Ie. if you make it so strong that it will not flex and it will not break, you want to be sure that it is not going to cause you harm that may otherwise have been avoided, for example lacerations from the edges of the part. One possible way to avoid that would be to flare the edges away from your arm ever-so-slightly, would only take a millimeter or two. Or if the foam liner that you mentioned earlier was thick enough, that might do the job too.