Here’s how to convert a plastic milk crate into a child’s seat with storage. Cut a square of ½-in. plywood slightly larger than the crate. Set a piece of 2½-in.-thick foam rubber on top of the plywood and wrap it with a canvas drop cloth or any sturdy fabric. Staple the cloth to the underside of the plywood to create a seat cushion. Now, set the cushion on top of the crate and store toys and books inside the crate.