I've got a heavy duty tarpaulin. As a would be showjumper mine is blue, but you could get a green or black one to make it look more ditchy. I love mine - can fold it down to make a really narrow tray for the babies, or make it wide enough for a proper open water. I tend to roll the long edges round a pole at either side to stop it flapping, works a treat!
I have a 'trolley cover' from work which was opened by mistake (operating theatres) so i got to take it home. Its basically a large blue plastic type sheet which is great - its strong rnough not to rip etc and it can be folded to whatever size you like, i simply put two poles either side to weight it down. Any blue tarpaulin type thing should do a similar job you just need to make sure its stuck down properly before you use it
My OH made me one - it is a wooden frame with hinges in the middle (for ease of moving it about), and a tarpaulin nailed onto it. Wood came from a local wood recycling project and we had the tarp anyway so all in all I think it cost about £20!