What thickness of Coroplast for enclosing TT underside?

You are probably using "styrofoam", generically, like "Kleenex".

My suggestion would be to use extruded foam board or polyisocyanurate instead of expanded foam board (Styrofoam). Both of those have more durability and higher R-value than expanded foam.

Since the chloroplast is only a shield and they all have the same properties, I'd opt for a thinner one.