I would a hanger but use something to block the holes on the bottom that way the adults can get the food there. This would use less material, especially if you have something that can block the holes (quilt batting?).
You can also use pvc
pipe to make a hook that can hook onto the side of the tank. Then drill a hole in the pvc
so you can ziptie it to the box. Also you should probably cap the end that is in the water to prevent fish from getting stuck in or from siphoning your tank on accident or something lol
Hope I helped!