As I wrote before, casting a port mold of a siamese port is not straight forward. Yes you can do it like you find on the web: stick in valve(s), pour silicones till full. And then find out that there is no way you can get the block of silicone out. I tested with squishy foam and even that is hard to get in and out of the port. Maybe with 10 shore hardness silicone you might be able to stretch it enough but I doubt you can pull it off as the mass in the siamese part is surprisingly large.
I read this : http://www.diyporting.com/molds.html. Permatex RTV cost a whole lot more than $8 a tube here ( more like 20) so I basically did something similar but used two (poly-condensation) part silicone. The sellers recommend using a 0.5mm coat with 0.01% thixo added ( one drop in 100cc). I brushed it on with a old toothbrush and after about one hour added a bit more (about 30 cc) and did a sort of roto mold in slo mo, . Uncured silicone is odd, and feels a bit like non-newtonian fluid. It get’s more solid when stirred vigorously but when you leave it alone it is surprisingly runny. I did not add thixo for the second coat and I think it is a better if you add a bit to keep it sticking more to the side. But anyway after a few hours ( 8 ) i could peel of the side quite easily ( I had put some dish-washing fluid on the surfaces as mold release) and blow the silicone away from the sides. As the mold is mostly hollow you can fold it enough to extract the whole unit from the runner surprisingly easily.
I have spend many hours peering down these ports but I have to say that I was quite surprised by the shape that comes out. The port mold is little dirty but i just cleaned the port as well as i could without grinding.