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.

Or - you could cut long strips of ply and glue then on edge to the back
of the board - if you could stretch to making the whole thing 3" thick
instead of 2, that'd stiffen it up a lot. Particularly if you ran them
crisscross interlocking.

--
Skipweasel
Pay no attention to that man behind the curtain.