Tony, you can use a piece of strip styrene of whatever thickness
you desire to correct droop when the coupler droops within the
pocket. You glue the strip (i.e. shim) at the open edge of the
pocket, so the shank of the coupler is lifted upwards. A little
experiment will show how much shim you need. Note this is only
for coupler droop -- if the pocket is too low, then you need
another solution. Obviously you glue below the coupler -- I've
never seen a Kadee coupler droop upwards. (Pocket tilt is a
different problem.)

Tim O'Connor