This is the shape I came up with, allowing for different diameters to be made. The little lips just help in holding whatever I’m adding in position.
So, how to make it in silicone? Well, I can’t make a silicone mould of this to cast a silicone mould, as the new silicone would just stick to the old. I could very easily just make a 3D printed mould of the shape by virtually ‘cutting it out’ of a solid block and printing that. But no, that would be too easy…….