For adjusting all 4 corners to be on the same plane (but not necessarily level…) could use a Mason Line String similar to ~24:15 in Neat! - #248 by azab2c ? Guessing sag in middle of two equally tensioned mason lines crossing over each other (‘X’ pattern) shouldn’t be much, and probably won’t matter even if they’re both sagging the same amount? Could do this for some janky MacGyver barely above the sloped garage floor setup?