Here is a photo of my second attempt..I am starting to get closer to what I wanted, and getting used to the technique, but I don't have time for a third attempt.
With this one I actually used old bar I had, fitted them into the slab mould and then poured the black base colour on top.
My soap accelerated a bit with this one which gave me more defined, clean lines but also gave a textured top to the soap which I don't like as much. It also meant I couldn't do as many patterns as I'd hoped.
Interestingly, on both this one and attempt number one, I am seeing some separation of the base layer from the underlying soap. The first attempt I actually did the slab, and the base and top all at the same time, except I used an accelerating FO for the slab so it was pretty much hard by the time I then poured the base and swirls. I think if I do this again I will make the slab and base all the same pour and then do the accent swirls on top.
For my first attempt I poured at pretty much emulsion because I thought I'd need the time, and as a result my colours sank into the white base a bit and ran together. I didn't get as clean lines as I was hoping. Then again the batter did move around more easily. I also think I used too many colours the first time round and I personally prefer the more simplistic two or three colours.