I always thought it was his hand. From another vent that it landed in possibly.
Wait, why does Bespin have ground? I thought it was a gas giant.
But yeah if the script says that, I guess we can assume that's what it's supposed to be. One major improvement that could be made then is to make it more obvious what it is.
Also, why do you need external sources for the shape of cloud city? There are several very nice shots that together show exactly what it looks like in the film.