The butterfly clamp idea sounds like a winner. If you have an integrated hinge along the front, then use a double ended butterfly clamp (or something similar - an idea is looming in my mind..) that won't stick out in transit, then you are sorted.
To avoid the balance issue, what is the problem with having a second small empty enclosure on the back of the lower cab into which you place copious ballast such as stage weights or sandbags?) Two to four 1/2 cwt stage weights on each cab would stop them toppling too much.
..d
