Your "sewing shed" is coming along nicely. When we built our outbuilding (combination work shop and sewing room but with a wall down the middle to separate me from the saw dust), I too was surprised that the cinder block foundation extended higher up the wall. So, I ended up painting the cinder block that same color as the walls and as it is covered a lot by sewing table and other "stuff", it worked out pretty well. My dh then did as you are doing by cutting and putting a wood shelf on the cinder block to cover it. We also have the combo heat/AC unit in each side. Bet you are going to really enjoy this room when it is done and you can get in there and sew!