I have tried several things for a solution to the "one person assembly problem" on casework.  I bought a couple of these Bessey clamps to try to assist in keeping casework square while I dry-fitted parts during assembly.  They work fine, and it's a real plus that they can deal with materials of different thicknesses being joined together.  These are typical Bessey quality -- nice, precise.  I compared them on the shelf to a similar product made by Columbian, but the Columbian was much heavier and had a cheap cast metal look about it.  It didn't seem as precise as the Bessey.