Any ideas?
I have made several chaff collectors for poppers using 1" x 1/16" aluminum bar and steel screen. Make 2 rings using the aluminum bar, one about 1/4" in diameter bigger than the other, and with the smaller one just a bit larger than your stack on your popper. Cut a square of metal screen about 16" square, and make a 'mushroom cap' shaped dome with the screen, with the edges of the screen going into the interior of the larger ring. Then shove the smaller ring inside to hold the screen in place. You can slide this over the chimbley when the chaff starts to fly, and remove it usually after the first crack is completed. Use RTV or rivets to hold the rings together (one inside the other, with the screen edges between them).