The following image will hopefully explain in greater detail my procedure.  I've recently used a product from 'Smooth-On' called 'OOMOO 30',  but had a disastrous result.  I must admit,  as good as it seems to be I just hate working with it.  Here's what my original mould box looks like with my plug secured in the base,  I then poured the silicon rubber into the mould box .........  it was an awful lot of work,  trying to mix what was a consistency of partly molten lava,  and the impossibility of pouring made the job needlessly difficult,  the result was a failed pour and a very badly formed mould  !  >>