Get a skinny bookcase. Usually found at Bed bath & Beyond.
Olive...Can you use a rolling wire shelf? or those heavy plastic drawers? Can get at Lowes.
I have a wheeled three shelved cart that fits perfectly between the w/d. Amazon has many choices in various measurements that go together easily.
These are also available on eBay too........
Build a rectangular box that fits between the machines. Nail or screw a back on it making sure the corners are square. The back will keep it square once it’s attached. Nail or screw shelves in it. By making your own it will be the perfect size. Finding one that’s just the right size can be nearly impossible.
Sounds like a good idea! Thanks!