I went and spoke with our boat service manager. He repairs inflatables and regularly casts them when needed. He told me first thing, casting is a last resort. He pours liquid latex through the valves. We charge about 70 bucks a quart and one quart is generally good for one chamber. We buy the latex in 3 gallon containers. It is a great solution when you don't have access to patch/repatch the leak.
Once inside the chamber and the valves have been retightened, you may need to flip the boat over a few times and spread the latex around. If the leak is in a spot where the latex can collect on top of it, then you can just leave it in that position. It takes about two weeks for the latex to fully set inside of the chambers.