Ok, I didn't see anyone else post this, but what if it's the Large flying droid in the droid factory that carries 3PO on a platform for awhile?
That's one droid that's huge and would fit the profile.
Thirty pieces to make it?
Maybe not, but other than the load-lifter, Homing Spider droid or tank droid, I can't see there being much else left. Besides, it is marked as being from the "Droid factory" so my suggestion has merits.