After trying to make individual forms for each hole and leveling between them, I decide to build a form I could use for each row of holes front-to-back. Thus, I could level the form front-to-back and know all 4 holes were level E-W--to the extent I accurately staked the forms before pouring concrete. Four holes took 1.5-2 bags and was about as much as I wanted to tackle in a day, so I could disassemble and re-use the forms from day to day.