Lots of options here - Home Depot, Lowes, etc. have a variety of shelving ideas. The plastic covered wire shelving can be cut to length (wirecutters or hacksaw). Some folks have installed it upside down so the downward lip on the front becomes a "fence" to help keep things in. There are the metal strips that can be attached on the two sides which allow adjustable shelving. Any of the shelving can be cut to width. There are also the little pegs and plastic hooks that allow the shelves to be adjusted - just add a board with a hole pattern or drill the holes in if hidden on the outside. When the adjustable shelves are not needed they can just be stored horizontally (flat) in the bottom.
Go
here and look at figures 13, 14, & 15 on the L/H side.