I just finished adding some casters to my Twin Reverb since we all know how light they are. I didn't want to drill holes into it, or worry about the cab having problems later on down the road. So here was my solution, and it works great! It does not interfere with tilting back either.
I went to home depot, bought some 2" casters, a piece of 20" by 9.5" poplar, some screws, some Velcro, and a can of black paint, about $30
Don't be skeptical about the velcro, trying to take the piece off the amp isn't easy, it takes both hands and effort to get it off (a lot like me!)
No harm done to the amp, no holes, if I ever decide to take it off, I'll only have glue residue to worry about.
Here are the pics.