Twenty years ago when we remodeled we made the area under the stairs
into a pantry. I had some shelving left over and put them in. After
a year or so we realized they were not deep enough so I added a six
inch piece of plywood behind the shelving and we had plenty of room
except that the plywood was about a quarter inch higher than the
shelf. I finally decided to fix it so with one sheet of plywood we
now have four nice shelf's and added two small ones at the top.

Looks and works great!