I made some tunnels for my brassicas. The blue stuff is water piping bought from B&Q. It is slotted on to bamboo sticks about two foot high, which keeps it in place. At the moment, it's only covered with netting, which won't keep out the butterflies. I have been looking for some finer netting which will keep both the butterflies out and the whitefly, but so far it has proved very expensive (anything from £30 to £48 for both tunnels, which measure 5 metres in length and need 2 and a half metres to cover them widthwise). Don't be fooled by the fleece on the other tunnel - it is absolutely useless and blows all over the place like a sail in the wind, and is full of gaps, plus you can't see through it and it channels rain over the sides. The only thing it's good for is warming up the space underneath it, which in temperatures of 25 C is hardly necessary.