I've just found a fantastic product on a building site. It is a sort of open honeycomb sheet, used to put against retaining walls to allow the water to run down the wall into a drain.
It makes for an ideal grating in any deck locker which has drains in the bottom. It stops the drains being blocked by the contents and keeps the contents clear of the bottom of the locker.
Called Versicell and supplied in 500mm squares which link up with each other. Easy to cut and fit into odd spaces. Strong enough in compression to happily deal with point loads from, for example, a full gas bottle. About 20mm thick.
It has solved my awkward problem of making sure a particularly locker under the cockpit drains freely - great feeling. I should say that I've no connection with the manufacturer - just think some of you guys might find it as useful as I have.