I have a CPS2 cab. The corners and the plywood bottom are both too weak for casters. This thing is HEAVY.
First I flipped the cab onto it's back with some help. Then what I did was cut 2"x4"s and screw/glue them onto the bottom one at a time. Be sure to screw them from the sides also for added support. I think I ended up using about 3 each for the front and back and then mounted the casters to the boards. it has held up fine that way.
For the first piece on each side I actually put the screws in from the inside of the cab and then put the screws in from the bottom for the rest.
Hope that helps.