A long time ago we made a fiberglass bumper for the rear of a '61 Falcon and it turned out great. Here's what we did:
We used the stock bumper and filled a few minor imperfections, it was pretty good. I took several layers of duct tape and taped over where the license plate went to make that area even with the rest of the new bumper.
We then waxed the bumper with car wax and set it up on some jack stands chrome side out. Using fiberglass resin and cloth not matting we carefully covered the whole bumper making a mold about 2 layers thick. After this cured we just popped it off of the old bumper. We had worked carefully to keep out the air bubbles and this made the inside smooth like the old bumper.
Next step we placed our mold on the jack stands again and waxed the inside with car wax. Then we took fiberglass cloth and put that inside of the mold piece. Then we built up the thickness using fiberglass matting. It took a whole gallon of resin and I mixed it up in one big batch. It took two of us one laying down the fiberglass the other handing him the materials because it's a sticky mess. Soaking the matting completely with resin and then laying it on using a little stick to get out the air bubbles, you have to work quickly.
It gets hot as it cures but fear not, if you make a mistake it's easy to just let it cure and then cut it off. Our bumper then cured and after that was done we carefully peeled away the mold layer leaving a near perfect fiberglass bumper. A little filler in a few spots and some trimming and sanding around the edge was all it needed to look great.
It was a scary project but it turned out a lot better than expected. We just tried it for fun to see if it would work and it turned out great I wish I had a picture.
Any sufficiently advanced technology is indistinguishable from magic --Arthur C. Clarke