If you are casting hypertufa, plaster or concrete in your new silicone mold you will need to build a casing for it.
If you only plan to use the mold for one casting you should make a simple plaster cover.
The fastest way to do this is by using plaster of Paris.
Mix it and soak some strips of cloth in it and build up over your mold.
You should go for at least ¾" thickness.
Don't worry about how it looks you will be cutting it off and throwing it away when you are done.
I probably should tell you to make sure that the master is in the mold when you start.
Remember that you will need to take out the master.
Be sure to leave some way to do this.
I can't tell you how because every mold is different.
Sometimes you can get away with filling the mold with water and making your casing around it.
Test this buy filling your mold and seeing if it stretches.
If it doesn't stretch the water idea will work.
The nice thing about water that it is easy to take out! You can take it out once you have built up to the point where your plaster will support itself.
Once it is out you can finish the buildup.
Keep in mind you will need a hole to pour the concrete into the mold.
You will also need a base on the casing so the whole thing does not fall over when you are pouring.
Generally the casing is set up so that you are pouring into the bottom of the mold.
This means that it will be sitting upside-down.
I guess by now you are starting to see a little more clearly just how much work there is to making a "simple" silicone rubber/RTV mold? Wait 'till you see my 'short' story about making a fiberglass supporting cover!