Leigh, one thing to keep in mind is that most silicones adhere to themselves very well. What this means is that you can put off the construction of partitions until after the whole sculpt is safely jacketed in a layer or two of rubber.
That is, you give the sculpt one or two coats of brush-on silicone all over to create a skin, then build your first set of partitions on top of that. Build up the rest of the layers of silicone against those partitions for one half of the mould (third, whatever) then when set remove the partitions, vaseline the edges of the first mould half - being very sure to get NO vaseline where you want silicone to adhere - and build up the other half of the mould.
Once all that's done, build the partitions for the glass jacket and make that.
When you come to pull it all apart, most of your mould separates along the partition line - peel the edges apart, and where it all meets the first skin you just cut the rest of the way down to the sculpt with an X-acto. You may mess up the sculpt a little at this point but it hardly matters.
When you see a cast piece that has flashing along the mould line which resembles a cut through rubber, it's because it's been done this way - the whole piece plonked into rubber, and the mould halves cut apart later on, to avoid claying up and casting the piece in two stages to save time or damage to the master.
Disclaimer: I'm just a hobbyist, every so often I still bugger up a mould, etc. If you can get a pro on board then do!!
