Turn the entire cage upside down. Cut four 2" long blocks from a 2"x2" beam. Glue each block to each of the four corners under the floor, as in the picture.
Let the glue set for a few minutes.
Turn the entire cage right side up. Using 1.5" wood screws, screw the blocks to the floor.
Turn the cage upside down again and line up each wheel.
Attach each of the four (4) corner wheels with four (4) 1.5" wood screws.
For the fifth wheel, I attached the wheel diagonally across the center of the middle floor beam with only two (2) 1.5" screws.