I thought the same thing, thinking that 3/4" plywood would be a GREAT insulator. When I moved the tank, the heat pad was just sitting on the plywood. When I touched the underside of the plywood, it was more than warm! When I touched the cabinet surface, it was toasty.
I know terms like "toasty" and "more than warm" are subjective. My laser gun is on it's way, so that will give me more precise description.