Some thoughts:
1. You can unitize the whole assembly by running a medium-duty black ratchet strap through the handles of the top cab and over the head, hooking the strp to the bottom cab handles. Don't block any air vents on the head, and use just enough tension to make things more stable.
2. Run your instrument lead though the handles of the top cab as well. Either attach with velcro or tie the cable with a shoe lace (with some slack) so that if you accidentally hit the end of the cable it doesn't pull the head off the stack.
3. If the bottom cab has non-removable casters you can use scrap lumber to make blocks to set the bottom cab on such that it can't roll on the casters. Some chunks of 4X4 (or 2 2X4 screwed together) covered with black felt or carpet, ends painted satin black - just slightly longer than the cab is deep.