A really concise & well executed 'fix', Steve. Looks good too.
In answer to the question re; plywood. Provided you used an exterior type which uses a waterproof glue, as you said & it's sealed, I can see no moisture prob. IMO...
As Ken has said, a kitchen polypropolene? cutting board would be suitable as well.
They are easy to work. ie; drill/cut etc... + it's cheap.
L.