Easily and simply add media to your filter. Seachem's The Bag is a great, versatile product made from mesh fabric of 180 micron. It's a fine and durable material that will last and last. It has a resalable closure too, which means media won't escape.
The bags 5" x 10" filter bag has a hard wearing non stitched seam which won't degrade as easily as other media bags.
To use simply add the media and seal the bag. Then add to a high flow area of the filter to fully expose the water to the treatment.