Bouncing ideas...

...you'd think with a rubber brain it would be easy to bounce ideas around,

Here is the silicon rubber mould.  It took some very gentle persuasion to get it out with damaging the plaster mould.  It's the first time I've used this specific brand of mix and I'm reasonably happy with the results although there are quite a few little nodules formed that aren't due to the original mould so must be a reaction or peculiarity of the mix.  It won't cause any issue as they'd be easily dealt with in any subsequent plaster mould.