Buy some modelling wax. It comes in different thicknesses, you lay that on the plug making sure its smooth then lay up the other half of the tool onto it. 

or, now that you have made the CF part nice and smooth on the outside, just wax it very carefully place it on the plug and make a new "female" mould on top of it..

Then you have a mould of both sides of the piece, and you can use the plug for compressing the wet layup in the new mould .  (I'm assuming you did this as a wet-lay without vacuum?)

If you have a vac kit/setup then just do a wet-lay and vac-bag in the new female mould..

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Well its not so nice and smooth from outside, could be so much better, so i dont think it as an option to make female mould from it. 

Easiest way to do it while keeping things simple is to put a layer of peel ply on before vacuuming it, be sure to lay it with no creases then clear coat the textured peel ply afterwards.




forgot to say you also want a layer of perforated film and then a layer of cloth to soak up the excess resin.