Use a mastic tape..not foil, not caulk, but mastic tape
(hvac supply)
cut lengths of mastic tape that will fit
all four sides of the supply box.
apply mastic tape to sheetrock ceiling, and fold into supply box to seal to metal of box.
press well to seal.

make sure that the tape is covered by the
supply grill. (mastic tape will tear sheetrock paper so
lightly trace with a pencil around the grill before removing, keep mastic tape within these lines.)

this will keep the supply box in contact with the ceiling.

what usually is occuring is @ that gap, hot attic air
meets cold supply box metal, condensation forms
and this causes the grills to drip.

I seal all supply boxes this way.
if one is installed where it doesn't fit flat into sheetrock opening..the rest are usually pretty much the same.

if the sheetrock needs minor cutting back you can use a razor knife.

best of luck.