Thanks for the ideas folks, particularly the damp proof membrane. Screwfix have it in 30m x 18" rolls for £15, so even if it took three thicknesses clipped together for rigidity, I'm still quids in!

Doganjo - Thanks for the offer of the boxes, but I'm using a big square electric hen, rather than a heat lamp, so it won't fit in a box. (It does fit across our bath at a push, but that's not really something I want to do again, as the droppings blocked the u-bend!

)
The other thing about the rings is I can set up a couple next to each other in the barn, and can just shovel and hose down when the bedding is soiled. Well, that's the theory at least - watch this space!!