I need to flip the mould design upside down, the disk with the encoder coupler should be placed at the bottom, cement poured on top and then the top flat disk placed on top as flat as possible, this is to introduce air bubbles into the top surface which looks a lot more interesting that a smooth or uncovered surface. If you look at the image below you can see the effect of placing a disk on top of the cement and in my opinion it looks way better. I am also considering mixing some black cement dye into the mix in order to produce a black knob which I think will look very interesting. (taken from DIYPerks youtube video which originally gave me the inspiration for this project).