After looking around online I finally found something that would work - textured plasticard with a cobblestone pattern.
Here is a close up of the pattern -
This stuff works perfect. All you have to do is cut out the plasticard to fit on the base, glue it down, and then paint it. For mine I used a black undercoat with a Dheneb Stone/Fortress Grey dry brush. After that I washed it with black wash and it is good to go.
Cutting out the bases was easy enough for the chariot base above but I ran into a challenge when I moved down to the smaller base sizes. I was able to easily and quickly cut out the bases with a little craft paper cutter though -
