Lee the main filler I was talking about the tri hydrate, is to extend the resin...lots of filler = thicker resin.... The cold cast stuff will be ally powder or brass in the case of your re-breather.
Moulding as a block mould is a possibility...But doing it as a single sitting means you run the risk of increased air bubbles unless you de gas the silicone or pour from a height, as this thins the stream of silicone and means bubbles form less readily.....From personal experience I'd do a brush up, using a coat of tinsil70-25 to pick the details up, but adding about 11% cat than the usual 10 to get it to kick a bit quicker and avoid the time consuming task of continually brushing the dribbles back on to the piece...After that do a layer with the normal amount of cat once it's cured, but add a good thixo to it. you want a consistency like melted ice cream and brush this on, it'll still run but will be more manageable. The last layer needs to be thick...like a peanut butter, so you can trowel it on once the previous one has cured to a degree. You can also add muslin or cheese cloth at this point, this will add a good mechanical bond between layers and also increases tear strength too. add you keys and then add a layer of thickened resin for a jacket. Sounds a lot of work and it can be but it will save you cash and extend the life of the mould too.