I use Green Debris Netting, its 1mm nylon mesh net used to shroud buildings with scaffolding round them. It doesnt stop rain or sun, but the mesh is fine enough to stop everything else, including carrot root fly. I make cages from pallets, 1.2 m x 2.4 m x 500mm , and my cabbages are pristine, not a single insect bite on them. You get the net off ebay, its about £27 for a roll 2m x 50m , and the stuff is practically indestructible, doesnt rot or degrade in UV.