I am in the process of building some french cleat mounted cubby storage boxes. The sides, top, and bottom are made of 3/4" plywood. I'm trying to decide what to use for the backing. I prefer to rabbet and recess the backs. I figure there are three options: hardboard (1/8" or 3/16"), 1/4 plywood, or 6.5mm (1/4") underlayment. What would your prefer?