I want to replicate an item using epoxy resin but I’m unsure about how to create the silicone mould because of the shape of the item.   The item is essentially a rectangular block of wood, say 44 x 36 cm and about 6cm high.   The complication is that there is a large cut out  that goes all the way through the piece from the top to the bottom.   So I know I have to make a mould box, but other than that I’m not sure how to proceed.  Should I position the piece in the box the right way up or upside down?    If the former then do I need to put some supports under the piece (which will be removed from the cast piece) so that the silicone can flow under the piece?   Where should I cut the mould open?

Any advice or tips will be gratefully received.