I added an extra step provided in the pack and another final base at the bottom of 1/2" thick balsa.
I painted the whole caboole with Dark Umber acrylic paint.
These are noticeably narrower than the other stairs will be but they look good and worn just now. I will be "varnishing" them.
I put a little piece of moulding as a little Roman a' clef to cover up rough edges.