1. Make a cardboard box big enough to comfortably house your object with sufficiebt space around it
2 lay a brick of non-sulfur clay at the bottom of it and embed your object half way in.
3 Use a pen or something to stab alignment pits into the clay.
4. Liberal amounts of mold release spray, mix your silicone well and pour slowly over the object from a hight.
5 let CURE
6 pop open the box, remove the thing, remove all the clay, flip it over, reseal the walls of the box, (now the silicone is on the bottom with half your object sticking out of it)
7 add clay snakes for pour shafts/vents
8 repeat steps 4 and 5