So IMHO use what the RW currently uses, which is LD3s and LD6s.
Very simple, larger aircraft, 757, 767, A330, etc carry the LD3's and LD6's. https://en.wikipedia.org/wiki/Unit_load_device
That wikipage has the basic info on the LD3 and LD6 plus what each cargo aircraft can hold.
And for sake of simplicity use the same size dimensions for smaller aircraft, 737, A320, E170, etc. And sense an LD6 is just 2 LD3s it would be very easy just to use LD3s. It is the universal cargo shipping method in the airline industry, at least for the past 30 years.