Here are pictures to show how I make flow-through boxes for fry. I use Sterilite brand boxes because they are fairly transparent. Water is dripped in from a distribution system at the opposite end of the box to the elbow, which drains the overflow to the sump.
The finished box, showing the elbow containing a sponge plug. These plugs are the "cut-outs" produced during the manufacture of sponge filters.
I use a piece of tubing into which the barbed elbow fits loosely to run to the drain. That allows me to lift the box off easily.
On the left, a 1/2" thread/barb elbow; in the center, a rubber washer; on the right, an elbow that is threaded on one side and slip-fitting on the other. The slip end takes the sponge plug.
Close-up of the assembled system. The washer lies between the inside elbow and the side of the box.