A novel idea - put your fertiliser in this small black plastic container. Fill this basket with any slow release fertiliser.  Height 22mm; Top diameter 23mm; bottom diameter 18mm.

The baskets prevent slow release fertiliser from falling through and out of the mix. They  contain the beads within the basket and when empty are easily removed, leaving no spent beads in the mix. When the plants are "benched" at shows the baskets can be removed to give a cleaner look. When the beads are spent, simply empty the baskets and refill. Can be reused numerous times.