A quarter inch orifice is an extremely sensitive flow measurement. But it will give you some sort of a definite baseline number with which to reference total air leakage. There may be more than one leak, so as you work your way around the bench, any small improvement can be readily quantified.
Mix up a bucket of laundry soap suds, and place great arm fulls of bubbles around the outside of the bench. The corners can be particularly leakage prone. But don't forget the panels themselves can also be slightly porous. But soap bubbles are a fairly good indicator. They will either rapidly expand in size, or shrink down to nothing fairly rapidly at the exact point of air leakage.