My first thought was to cut out individual bricks and attach them. I used this method on the back wall but I was not satisfied with the results. Cutting out the "bricks" was also tedious and messy. The I hit upon the idea of making panels of styrofoam, attaching them the the walls and textured with an xacto knife. I cut a piece of styrofoam about 100mm x 30 mm then carefully sliced off a 3-5 mm section. I continued making panels until I had what I needed to go around the chapel. The panels were glued to the lower part of the wall, clamped and left to dry overnight.
When the glue was set I took my xacto and carved the stones. An old paint brush was used to clean up any loose bits of foam that stuck into the crevasses. For the door I fashioned an arch with left over bits of foam. The door was scored and the wood effect will be brought out in the painting phase. Some tiny nails and craft sticks were added for details.