Gluing bitumen at the bottom sides.
https://www.buildandlisten.com/loudspeakers/cno-grand/buildprocess/1bd77403-e50c-44c8-9ae3-e6b56b333cb0.jpeg/view
https://www.buildandlisten.com/loudspeakers/cno-grand/buildprocess/1bd77403-e50c-44c8-9ae3-e6b56b333cb0.jpeg/@@download/image/1BD77403-E50C-44C8-9AE3-E6B56B333CB0.jpeg
Gluing bitumen at the bottom sides.