Now that my silicone RTV rubber has arrived, I can put the master part in the box, and seal its edges with modeling putty.
You have to seal the edges of the piece (unless it is perfectly flat, and even then I would still use clay), otherwise the poured rubber will flow underneath the piece, and you don't want that to happen for a 2-part mold.
In this case, I am using an oil-based modeling clay from Van Aken, that never dries out, and is compatable with silicone rubber.
That last part is important - the clay must not contain sulphur, because sulphur can interfere with the silicone rubber's curing process.
The stuff I got is pigmented (colored), and in retrospect I should have gotten pigment-less clay, as the pigments leave a mess on my hands, and the oil-base is a bit of a pain to wash off.
There was also wax-based clays available, but they didn't explicitly say "silicone compatable", so I passed.
The picture shows the master part mostly sealed.
I've applied a 1/4"-1/2" snake of putty to the edges, pressed it in to seal, and trimmed it with a special modeling razor blade.
The blade that I used was a Flexible PolyBlade from AMACO (American Art Clay Co), made of super-flexible stainless steel 6" long, and can shave off slices of clay super thin.
I got it from Michael's arts and crafts for about $5, well worth it. I also use it shave off thin slices of posterboard, but I wouldn't use it for heavy cutting.
The rest of the part will be sealed off, and the casting and venting channels will be created out of more modeling clay, to be cast into the rubber as channels.