We had a great run through most of summer free of white butterflies (and their broccoli-eating caterpillars) but that run ended. This is our attempt to keep the butterflies off our tender brassica seedlings and I have to say it worked out pretty well for an impromptu solution.
We used some old bird netting, some spare fence posts and the wire hoops that came wrapped around bundles of posts. Straw bales hold the ends down, making it (so far) butterfly-proof. We might have to track down some more netting to do this to more beds next year.