A stainless steel cylindrical wire mesh open at the top that is approx 15mm narrower in diameter to the brew kettle and just as tall. Simply slip it in before boiling hops, then when finished with boil pull it out collecting all hop and some break material. Easy!
The hops would still be free to roll around, but would the mesh basket prevent proper boil? Can't see why it should, as long as the mesh holes aren't too fine.
Ready now to be told it has been done or won't work
