I'd think of using a rack to mount them, then lower the baskets until the tops are at the level of the rack. Then you have a flat platform for large, boxy items.

Another thought... since you have the rack/ tops you could put a little padlock on them and make them lockable! Super cool.

Also, small stuff will want to roll around. Run some mini-bungie cord from the middleish part of the bottom to the sides so when you put stuff in they cinch down a bit and keep stuff from rolling around.