I made these shelves to fit between those two pilasters. These shelves were 1 1/2" thick, made of 2 pieces of plywood laminated together. I scribed the back of the shelf to fit nice against the wall. I left a little extra on the back that was milled at a 45 for easy cutting, hard to describe without a picture, but I wasn't on planning on making this a diy tutorial.
Here's a little action shot for you
