My suggestion would be to make a two part mold; make a small box that leaves about a half inch between the wall and the piece you want to cast, fill it up halfway, press the mask into the silicone, and let it dry. After it’s dry, cut off the excess silicone inside of the mask, touch a rod of some sort to the mask (make sure that it’s wide enough to let whatever material you’re casting flow into it), and fill the box the rest of the way up. Once that dries, you should be able to carefully pry the top half off and remove the mask, and have a decent enough mold. I would ask this guy, as they’re definitely the best mask molder around at the moment.
Thin your paints and apply a lot of layers. It takes longer, but leaves no streaks. You also might want to check out this topic.