Reading the June newsletter tip on weed free brassica cages, here's how I do it. A system perfected over many years! (And still being tweaked)
My brassicas follow my peas. When I plant out my peas from gutters I lay sheets of cardboard between the rows and cover these with wood chip to prevent weeds. Once the peas are finished, brassicas are planted in the pea rows & the card & woodchip stays put. By the time the brassicas are finished by the next Spring the card has disappeared & the, by now part rotted, woodchip can be dug in ready for the next crop which is probably carrots & roots.
I got a gazebo from my local recycling centre for £2 (actually, I bought two). Mine is made up of several poles which slot together and I use these for the uprights. You will need to buy four aluminium poles to form the top of the frame and some plastic three or four way joiners, all of which can be found at garden centres or online. The initial frame can be build using one section of gazebo pole in each corner as uprights to start, adding another section when the plants grow taller. The whole is then covered from the onset with protective mesh against Cabbage White butterflies. The beauty of this system is it's cheapness & the flexibility of altering the height of the cage as and when required. I also use the card and woodchip mulch between my sweetcorn rows which allow the butternut squash planted underneath to ramble unchecked by weeds.