You need to add a small filter (like elite mini) that will pump tank water into the box and the holes in your box will let the water. The shrimp need a regular supply of clean oxygenated water, the water standing in your box will go stale and your shrimps will not live for long.
BTW how are you going to clean the leftovers from the algae wafer?Everytime you will have to remove the box out of water this will stress the shrimps. I hope you know that food should be left more than 1-2 hours in a shrimps aquairum as it starts to rot leading to rise in nitrate and Planaria, tubifex worms, etc.
I can only advice try to move them to a shrimp only tank if you truly want to enjoy your shrimps or buy a satellite 2L breeding box.