For plywood sheets, stack as many as possible and secure with ratchet straps. Rig from all four corners to the central lifting point. Make sure that the strops are equally loaded otherwise the load will oscillate horribly. This works fine but take it easy as obviously a horizontal "slab" load can always give problems.
The problem with cabins is securing them, particularly if they are ply. I have tried various ways of rigging but the best always seems to be a net if it is feasible to get them in and more importantly out again.