After my old fry hatchery fell apart due to rough handling and old age, I needed a new one for the killifish eggs my son and I are hatching, as well as for potential cherry barb additions. Here's what we used.
1 2" tall round tupperware (the clear plastic sort you get salad bar stuff in)
1 pair nylons
length of aluminum bonsai wire. NOT COPPER!
1 1/4" wide rubber band, smaller will work, but not too small. It will need to go around the tupperware top to secure the nylon
bubble wrap (optional)
Cut the bottom out of the tupperware.
Cut one "foot" off the nylon pair (should either be brand new or well washed) about 5-6" from the toe.
Taking the bonsai wire, create a circle just slightly bigger than the hole in the bottom of the tupperware.
Start by placing the wire ring inside the nylon with the seam on the toe to the outside. Push the nylon up through the bottom of the plastic container and cinch the nylon tight before securing it around the top of the container with the rubber band. Make it tight enough that there isn't too much slack for the fry to get caught in, but not so tight that it will pull itself apart. The ring should be holding the bottom open and keeping it from slipping up through the tupperware bottom. To create a floating fry hatchery, wrap bubble wrap around the outside of the container and rubber band into place. To create a stationary "hook", bend the bonsai wire into a second ring with a nice long tail. Before putting the hatchery inside the ring make sure it is big enough to go around the outside of the container but tight enough to hold it up and then crook the tail end over the edge of the aquarium. Adjust the ring and then insert the hatchery. Make all adjustments before adding fry, eggs or momma fish. Tada!