I have used a foam rubber floor matt with the area of the handle cut out. Also helps to isolate the cab vibrations from affecting the tubes in the head. (6L6 usually are quiet but EL34 are not and typically rattle).
Taller feet or something in between may be good. If you do not move your gear often, you may be able to remove the handle. If you do, put the screws back in the holes but do it carefully otherwise you may push out the threaded inserts inside the cab. Taller feet may be a better option.