Have you thought about making precast concrete cap stones? Say six pieces, one for each side. Miter the corners. Overhang the inside and outside edges about an inch or two. Make the forms out of plywood and 2x4's. Mix up some concrete and throw in a couple of rebar. If you wanted to get fancy you can add some color to the mix. Depending on how symmetric your walls are, you might only need one form. I've never done it myself but seems like it might work. There are several videos on YouTube.