With my wooden mould complete I was able to use the vacuum former to create a plastic shell that matches the shape of the owl heads. I'm going to cast that shell with a silicone compound to make an exact replica of the wooden mould and I'll be able to make two shells from a single sheet of material.
I used a large block of foam to fill some of the void in the middle of the cast, this reduces the amount of silicon I have to use. Last time I tried this though I forgot that the foam would float in the silicon. This time I laser cut a base board for the mould, once filled the foam will float up against this and be held in place. I put some pour holes in it so that I can fill it from the reverse, but due to a combination of the foam obscuring the holes, the holes being in the wrong place and me being generally rubbish at pouring I ended up making quite a mess of the board. It was all good fun though, you can see that the silicon copy came out well and has been already pressed into service (cornflour makes an excellent mould release)