This is one of several converted chariots that I created for my Tomb Kings army.
I sculpted the "blanket" for the horses out of green stuff, which was then cast once cured. This meant that I could make more of them in a lot less time!
To make the cast, I smushed some green stuff into my mould, and placed a sheet of tin foil over the top, cut to size. Not only did this allow me to get the green stuff out of the mould more easily, it let me position it as I wished. The tin foil gave it stability as it cured, so that it would not sag, etc.
It also gave it strength while I was painting it, so that it didn't move around with each brush stroke.
I believe this was inspired by the image on the cover of the Tomb Kings book, which showed chariots pulled by horses with this kind of blanket.
The chariot and crew are also here: