I did one of these a couple of years ago. (my original build thread is
here)
Mine's about 7 or so inches tall, with two pieces of steel piano wire as the verticals, greenstuff for the rope rungs, and instead of a cloud, I did a strip of girder across the top, to suggest the hatchway of an airship. The base is a 1.5" washer, and the final version had a boulder and tuft of grass hiding the ends of the steel wire near the ground.

Not the best photo, but it should give you an idea. It's a fun piece, it's served as a spawn point for sky pirates in several games now and always gets comments!