First, if you stick with the milk crate put plywood on top. The plastic grating can crack and give way.
When we had a TC we had a drop down step, nbut based on the site, sometimes we needed an extra, picked up a step with four foldout legs to make up the difference. We had a rubbermaid style for a bit, but it was notorious for flipping and wind moving it.
I had friends with the accordian style sides that hooked on, but I never liked those.