On 28 Nov, 20:28, Skipweasel wrote:
In article ,
says... Bright ideas please.

Find a building site and scrounge an offcut of polystyrene or other
foam-type wall insulation. A 4" slab of that will make a nice lid, help
prevent freezing /and/ be rotproof.
--
Skipweasel.
Never knowingly understood.

Use Marine Plywood