Shape up the fuselage with a “Stanley Surfoam Rasp” or any coarse rasp and 80 grit sandpaper the first layer will not be pretty or perfect but just mix up some more filler and fill the dents and fair to the required shape. If you use 45 min set filler it goes off fairly quickly and you can work on it within the hour and add more as required. Now you can also add  detail like raised panels etc but remember not to make reverse or compound curves which will stop you getting the plug out of the mold. When satisfied sand with a fine sandpaper. It should now be like photo #  5

           

At this stage you have to decide what your ultimate goal is. If your fuselage is just a tube like shape and you only want to produce one finished fuselage you can lay the final fuselage over the plug and not make a mold. If the fuselage is a complicated shape as is the Vixen you will have to make a mold as you will not get it off the plug. If you really want to get serious and produce a fuselage with all the rivets panel lines etc molded in than you will have to skin the plug with glass and finish it to a high standard before making the mold as the plug made of just the filler is to soft to retain the lines, rivets etc.
 If you are carrying on with a mold from the plaster plug cut some 18-20swg steel sheet into 2” strips about 18” long and tap it into the joint between the 2 profile pieces screwed together at the start. Put them into the gap all around the top, bottom, and the sides this is why it is important not to screw these parts too close to the edges when making as it will make it difficult to get the steel strips in. These strips make the separation and joining flanges for the finished mold and is a lot easier than trying to make separation dams out of cardboard plasticine etc.
 Paint the whole assembly with and undercoat and topcoat. I just use lacquer it works well and is fast to use. Finish the paint to your requirement remember as with any mold  the surface on the plug is what the surface in the mold will be. Photo #  6  shows the plug with separators and painted ready for laying up the glass
The plug must now be “waxed” with release wax 3-4 coats is needed to ensure the mold will separate from the plug. No short cuts at this stage or you may be starting again. I also use a PVA parting agent over the wax before glassing just as an added precaution. I always use a gel coat when laying up a mold it allows the sanding out of any small imperfections in the finished mold. If you are serious about producing quite a few fuselages out of the mold use “molding gel coat” if it is required for only a few normal gel coat is quite okay. Glass up the mold with “chopped strand mat” depending on the weight and size of mold 2-3 layers are required. Make sure you glass up the separation strips. When dry drill through those separation strips about every 4” with a 3/16” drill . These holes are used to reposition the parts of the mold back together when joining up the various parts.  Glassing finished Photo # 7

 If you did the waxing etc right the molds should pop off by pushing some small wooden wedges either side of the steel separation strips. Remember to drill the alignment holes before you do this it is hard to line the mold sections up later without them. Photo # 8 shows a complete set of molds