Ours is also an open shelf in that space, probably about the same size as the OP specifies, and we use those soft fabric collapsible cube boxes you can get at Target and Walmart. We keep folded clothes and towels in them. Six of them fit very nicely across the whole shelf. Neither the cubes themselves or any of the stuff we have in them has ever moved around during travel. They work out very well. We have a "nightstand" on each side of the bed, so we each have an additional cube there for miscellaneous stuff. (And the cubes themselves fold down nicely in the camping bins for winter storage in the house.)