For the top shelf, cut two (2) 2"x2" beam to 1' long. Measure from the center beam to the top beam and cut two more 2"x2" beams to fit vertically between them. Separately measure the beams for each side of the cage, since the long shelf will cut about 1/4" off of the rear vertical beam. Lastly, cut a 2"x2" beam to 21".
It is helpful to use a 1' beam to measure the placement of the vertical beams.
Attach the vertical beams to the front and back using 2.5" wood screws.
If you have already tiled the long shelf, you will need to cut out a square of tile to allow the vertical beam to have contact with the wood underneath.
Choose a height for your shelf. Mark this height in wood pencil on all the verical beams. Then attach the horizontal beams using 2.5" wood screws.
Cut a piece of plywood to 1'x2'. Cut 1.5"x1.5" squares from the two corners that will be against the wall beams. Place the shelf and secure it with 1.5" wood screws.