I have been following your progress on the church. I think this will help. Start with a clean surface. Professionals use glass. I use wood. Paper will work. Attach the wall to the surface. Coat the surface of the wall with the latex mold material. You can use your fingers. Make sure you extend the latex boarder for at least one inch. Use at least 3 coats. Allow one hour between each coat. Some people add cheese cloth at this point to make the mold stronger,This is optional. Let it dry over night and add 2 more coats.
Now you need to build a box around it. Then pour the plaster back up. This is the critical part because this what supports the latex and keeps the mold from stretching out of shape Make sure this pour is level because the wall castings are thin.
This mold is ready for the box to be built around it. You can see the latex boarder around the out side, The wood helps to keep the mold square and level. When I clamp the box around the block I won't have to worry about leaks.
This photo shows the latex mold, the plaster casting, and the plaster back up. This is about the limit for a one piece back up. The deep eye sockets make the castings difficult to remove. If I was going to make more than a hand full I would need to make the latex thicker and separate the back up into at least two pieces
This is a commercial mold. This is thick enough not to need a back up.
If you decide to make a mold for the church, I will be happy to coach you through the windows.
