"The missing keystone, the bane of our efforts. We spent the better part of a day trying in vain to chisel or crack a brick into the appropriate shape. Turns out that bricks aren't particularly easy to cut cleanly, except maybe with a masonry saw (which we did not have). In the end, we took two sort of okay-sized chunks, jammed them in there, and hoped the mortar would hold it together. It more or less did, but it looks kind of kludgy and gap-toothed."