Guy King wrote:
The message .com
from "sm_jamieson" contains these words:

If 18mm ply is not stiff enough, is there any other board suitable ?

Ah, right, now I see what you're trying to do.

You could get away with 3/4" ply if you could stiffen it. A welded steel
frame would do nicely, 2" box round the outside, 1" angles running side
to side, with some steel noggins welded in between, the whole thing
screwed to the back of the ply.

Thats the kind of idea.
Edges will be screwed to battens anyway. Some kind of steel structure
sounds a good idea.
Cheers,
Simon.