Reply #3: i think your idea would work, really.

[mopinko]

if it was a good heavy duty box, and you mix the concrete nice and thick so it has a nice low slump. maybe you could brace up the box with some bricks, or blocks, or just wedge it into a corner. or wrap a couple loops of duct tape around it. you are not talking a half ton of concrete.
do oil it, tho. wd40 would be fine. helps slow water absorption, and makes sure the box will peel off.