Les,
I used a plastic casting resin called EasyFlo 60 ($22.50 pint) because it is thin as water and has some flex when cured. I do not like the color. I used their liquid dye to make it black. I think I will try some waste toner from a copy machine to color the next try. I wanted to use Plastic Epoxy as it has a very shiny finish; however it is also very brittle when cured and is thick to pour into a mold. I used the waste toner to color the epoxy in the past and it works fine. If the butt plat has a flat side next to the stock, you could cast it in a open top mold and let gravity smooth it out. The ZG 47 has a curved butt plate and I could not sort that out without using a closed mold. I used Silicone and blue Dawn dish soap to make the mold.There are a number of videos on YouTube showing how to do this process. Use plenty of Dawn as it is the catalyst. I used about 48 OZ of ice water and about 8 OZ of Dawn liquid. This was probably too much Dawn, but if you use too little you will have to work the silicone longer and it may cure very slowly. None of the videos I watched gave a ratio, but one guy did not use very much and had poor results. I glued the butt plate to a magnet which I stuck to the base plate of the mold form I made. My first one was square and I ended up with a mold that looked like a small loaf of bread. After I cast the Silicone around it, it cured in a couple of hours. I then cut the original butt plate (from another ZG 47) out with a razor knife using zigzag cuts to create registers in the mold. I did consider making a two piece mold, but this was easier. Some things I learned:

I used Pam cooking oil on the butt plate as a release agent, car wax would work also as I used it on the mold. I am not sure you need it, but better safe than sorry.

When adding Silicone material into the mold you will create small "seams" between the layers which will show in the casting.To prevent this you must stir the material as it is added to the mold in order to mix it better. It would be best to add as much to the mold form as possible as one piece. I will use a round mold form next time, fill it will the treated Silicone, stir and then push the pattern butt plate into it rather than putting some into the mold, adding the pattern and then more silicone. I will hot glue the pattern onto a small dowel rod and then push it down it the material end first.The dowel will make the port to pour plastic into the mold.

Pour your casting resin in as small a stream as possible to prevent trapping air bubbles. The Epoxy will be a challenge in this regard. I used .95 Oz of material to make the first plate and had a small amount left over. I think .75 to .80 OZ will do it. I used my digital reloading scale to get accurate weights converting grains to ounces.

John