you right. a dummy object is the best solution. You should place something like one or more little cubes or shperes (maybe done with polustyrene) on the edge. so you can see them from the two sides. this will help you a lot.
If you don't want to hide the small portions of the objects where the dummies should be placed, you can choose the second way: put the object on the floor or on a basament platform and then move near your problematic side one or two simple objects, like cubes or simple solid shapes. Scanning all around and be carefully don't moving the object and the dummies, you can have a "non ivasive" reference for the alignment.