hey simon
I've helped build a few over the years, We used in this seasons kicker
Builders ply for the top & sides easy to work with & the boards slide over it ok with a bit of silicon spray on them,
Ali frame,
a sheet of foam from an old cool room about 20cm thick,
A set of wheels off an old kids car that was been thrown out,
All this stuff is fairly easy to get (ours gets burnt/smashed fairly often

so this is a good thing)
Ours stands about 110cm out of the water start sits just under the water with a smooth trasition from the water.
Remember build it light, Ours can be moved by 1 person thanks to the wheels.

booga