Discovered this by accident when I got berley without the bag. From your local ladscape/garden supplies you can buy netting to cover strawberries. The mesh size is about 1cm x 1cm. Cut it into whatever size you like, I use a square metre for a 5 kg berley bomb, simply wrap the bomb in it and thread your rope/string through the mesh to tie it up. The mesh is very strong and doesn't rip. You can make the hole size smaller simply by wrapping more layers on. I find that just one wrap works well and it allows the biger bits to get out and not clog the bag up.