On Mon, 15 Jun 2009 16:32:28 +0100, Mike wrote:

Cut some thin sheets of Shuttering Ply

How long will that last in the wet? You really need proper WBP (Water
and Boil Proof) ply. Ordinary ply will fall apart in very short order
outside. Shuttering ply is a cheap use once and discard material,
even if the concrete is wet when put in the board doesn't have to
survive for long, just a few days until the concrete has mostly
cured.

I agree a thick bit of wood is the best solution. Visiting a proper
builders merchants or timber place will be better than looking in the
DIY sheds. You can probably get 4 or 5 m lenghts of rough sawn
treated 6 x 1 timber from a builders merchant, maybe even with free
delivery if you are willing to wait for the day the truck goes out in
your direction.

--
Cheers
Dave.