I was wondering if anyone has any advice on how to make slabs for group and single plantings? I have seen many people use slate, but these are both expensive and heavy

Anyway, I googled how to make a slab for bonsai, and I came across something called "Hyper-tufa". Having read a little about it, and watched a few videos on how to make it, it seem simple enough. My question is though, has anyone got any suggestions on how to make a mould for a slab? Or is there a better alternative to use rather than hypertufa?
I did a search of hypertufa on Ausbonsai first, and Flybri seems to be the only person to mention it, but in regards to making fake rocks for root over rock and rock plantings. This too seems like a great idea.
So I guess I am wanting to know how to make a slab. Is hyper-tufa the best/lightest/strongest medium to use? What would be recommended to colour it to look like a natural slate slab? Any suggestions would be greatly appreciated especially regarding making a mould for the slab.
Cheers
