Originally Posted by AmyKidsCo:
I have not, but I used pegboard to replace the cardboard back on two of those cheap shelf units, and it worked great. I'd use pegboard over plywood because it's thinner and lighter. (Maybe cheaper too, but I don't remember)
I second the pegboard suggestion. Then you can use the backs to either hang things from, or we have velcro on ours for various activities. We got ours from Home Depot, and they even cut them for us.