Roundhouse shell

The roundhouse shell is complete.

Some work benches and tool cabinets are in place

So far, so good. After looking at the photos for a while, I decided that I'm not happy with the look of the exposed plastic. Never would have thought that I'd get this anal about the inside of a building that will be located towards the back of the layout room.

There needs to be an easy way to simulate a somewhat dirty off-white brick wall for the inside of the roundhouse. This would have been easier with the walls off, but I figured it won't bug me that much. MEEEEK! ... Wrong.

At a minimum, I will just paint the inside red walls with some off-white color and "dirty them up" during installation of the roundhouse tracks. No brick texture, but better than these red plastic panels.

What are options to get some texture on these walls? It needs to be easy to cut and install, and should take paint well (if not in the right color already). Some kind of wall paper, maybe?