It should be similar to making a resin casting. But you'll need a good mold release agent. Something specifically made to keep silicone from bonding to itself. You'll also have to really be careful to avoid having air bubbles in the casting. You might have to do it in layers to prevent this.
What you seek is Pattern Release 202
Ideal for patterns and coatings used in the preparation of silicone rubber molds without inhibition. It can be used on a wide variety of pattern materials (wood, epoxies, urethanes, polystyrene, plaster and rubber) to name a few. Available in an easy-to-apply aerosol spray. Maintains excellent surface detail. Air dries with no buffing required. in short its a sprayable petroleum jelly
and will not let silicone stick to silicone if applied properly
I use this all the time when making two part silicone molds
S &H - Freeman charges standard published UPS/FedEx shipping rates (unless freight is specified). All orders under $25 are subject to a $5 small order fee. International orders are subject to a $250 minimum order. there will be a HAZ-MAT fee added to the shipping