Slick,
This was my post from last Sunday.....maybe some of this info would help !
1. Select a nice Specimen, no freezer damage, missing scales etc.
2. De-slime the fish. (dish soap and a tooth brush works. Never go against the scales.)
3. Prep. the fish. ( I remove the fins & cast them separately. If needed fill the belly area with caulk, Using a thin clear tube on the end of the caulk gun, and inserting thru the anus. Also, I remove the eyes & pack the sockets with a little clay.)
4. I use an adjustable mold box (from Polytek), to the proper size for the fish I am molding. The link below will show you some of the steps I am describing.
http://www.freewebs.com/aquaticreproductions/featurepage.htm5. For bedding material, I mix up some dough / clay. (Flour, salt, water & cooking oil....mix to a soft play-dough consistency).
6. I apply a little vaseline on the side of the fish that is going into the clay to the "parting line". (This is Important, because if any air is exposed to the skin it will cause wrinklng, flat spots etc.)
7. Now I place the fish in the soft clay, up to the parting line. I make "key" impressions into the clay, all around the fish. Then I spay an mold release on the inside of the box only. (cover the fish while doing this. I do not put anything on the side of the fish being molded.)
8. I use Reo - Flex 40 from Smooth - on for a "Flexible mold". For a 18"-20" trout, I will use a 1-gallon kit for the 1st pour and another 1-gallon kit for the 2nd pour. It 1:1 ratio follow the simple mix directions.
9. Pour the reo-flex 40 over the fish, and let dry for approx. 3 hrs. (it will not be finished dry....it take 16 hrs., but you can carefully handle the mold half.
10. Flip the mold upside down and remove the clay. I also remove the fish and Re-hydrate it, placing it in a wash basin and soaking it in water. Let it soak for 15-20 mins. while you are removing the clay. Remember to clean off the vaseline from the side of the fish that you applied it to.
11. Now apply vaseline to the entire 1st half mold (also where the fish impression is. This way if any Reo-flex gets under the fish on the 2nd pour, it will not stick)
12. Dry the fish off, and place it back in position in the mold, making it ready for the 2nd pour.(Do not place any vaseline or mold release on the fish).
13. Mix up the Reo-flex 40 for the 2nd side and pour it on. Wait approx. 3 hrs. and open the mold....carefully, as it is not finished hardening. (but should be safe to handle). Remove the fish and lay the two mold halves on a counter to dry over night.
This will give you a mold that can be re-used. It will be very easy to demold your cast as well. Just flex it a bit and the cast will pop out. In regards to the mouth area, you can cast it separate as well, or fill it with clay and sculpt the cast in by hand.....maybe this would work for you !