Prepare beams for the bottom shelf as you did for the top shelf. I used a 1"x2" beam for the front and back support of this shelf because I was running low on 2"x2" beam. Don't worry. This is pleanty of support. Use a 2"x2" beam for the side wall.
Cut a sheet of plywood to 1'x2'. Cut 1.5"x1.5" squares out of the corners that will be against the wall beams. Place the shelf and attach it with 1.5" wood screws.
At this point, your cage should look something like this.