Not quite the just paint up option, but grab some of the foam of about the thickness you want the wall, grab some of the textured wallpaper that look like it's stone blocks, and some cheap foam core, along with some gravel/sand.
Cut the foam to the height you want your wall to be, attach a piece of peeled foam core that's slightly higher to one side at double thickness. Cut crenelations in the foam core, then clad the sides and top with the paper...for broken parts, just crack the assembled sections up and add detail as you wish with a knife, then add the paper and glue a pile of the sand and gravel to the exposed areas. It's cheap to make and you can crank out a lot of them rather quickly.