Don't use a square mold...use a circular metal ring template much like one would use for a double ring infiltrometer test or like the heavily angle ring shown in ASTM D5030. The square molds are "suggestions" in the standard...easier to use a round template.
On one of our projects (not one I worked on), they apparently got squirrelly results using a large rectangular frame, so they went to a ring and the results made more sense.
