I have tried a few different methods.
One was backsplash tile on 1/4 inch hardy board. Looked great. Just be sure you use a tile with a rough texture and seal it with something like pond armor.
Another method, which seems to be most popular, is to get a sheet of foam (polystyrine/ white kind) and carefully cut away chunks and or carefully melt it down with a heat gun. Then coat it with tinted drylok.
One idea I had was to tint some non-shrink grout, pour it on to a 1/4 sheet of hardy board, shape it, let it dry and seal it.
Others here have made some great ones.