I've been using plastic canvas with great success for my grindal worms. Plastic canvas directly on the substrate then I place soaked pellets, wafers, whatever on top of the canvas. The worms swarm the food and I harvest them them with tweezers from the side of the food.
My whiteworms are being difficult. I use the plastic canvas but they don't swarm the food from the sides, they stay in the soil and just feed from the bottom. I've tried the 'piece if plastic' technique but it never works very well for me- I end up with too much substrate mixed in when I harvest and that just adds dirt to my tanks.
What are the rest of you doing?
20L My Other Tank: OEBT Shrimp
40B Inlaws Tank: 2 goldfish
75 Main Tank w/ sump: 5 Glossolepis pseudoincisus, 5 Melanotaenia boesemani, 10 Melanotaenia praecox, 5 Melanotaenia lacustris, some cories, a bn pleco and rainbow fish fry