It is easy to do. My cousins built Morgan yachts in Oldsmar, FL so he had all of the fiberglass resin and chop and glass with different weaves you could imagine. He helped me make my own molds for my hood, fenders, and inner fenders, trunk, and bumpers since no one made these parts for a 1977-'90 B-body. I made an outer skin for the hood and the inner braces (since I wasn't using my old parts any more I just took them apart by cutting the spot welds). I then was able to build up the hood to be as strong, well to stay on without being blown off and still save a third of the weight. Inner fenders were so thin they were translucent. Outer fenders were thicker on top and where they mounted but thinner elsewhere.
I could have molded in any hood scoop or bulge I wanted since it is easier to modify a plaster mold than sheet metal, but I wanted a light weight flat hood. Things like holes where the spot welds were left dimples sticking up that a three inch Scotch Bright pad on an air grinder removed in seconds. getting the surface smooth was easy since you could rotate the fixture holding the mold in three dimensions so that the area you were working on was always level.
In retrospect without the shop equipment that my cousins owned it would be more difficult but he made all of his own tools and fixtures by welding together pipe frames using one inch bearings to allow it to rotate in any axis. He used three quarter inch sheets of ply wood with two by four or two by six wood as a skeleton for backing of the molds to keep it from cracking while you worked on it. All of this you could do at home with enough room. The huge shop and twenty ton overhead crane where a plus on moving things around to shoot chop on top of the gel coat but ladders and maybe a scaffold could do the same in your garage.
These were one of one pulls so once I got a part that worked I allowed him to trash the plaster and reuse the backing of the molds for another project we worked on involving a Ford Econoline cab and a White truck chassis to build an almost all fiberglass camper with a lot of Morgan Yacht interior parts. We built three but they didn't sell well enough to cover production costs, that and he was deported back to Canada as an illegal alien stealing jobs from Americans; even though his shop employed over twenty people who lost their jobs when the business was closed down.
I learned a lot doing that, but I am still no body man. I thought of it as building a full scale model kit. And like my 1/24th inch scale polystyrene cousins I still had runs in the paint.