Most ceiling support boxes are square with smooth sides. What I've done in mobile home installations is to
cut the length of the box down, leaving the sides 2" longer than the thickness of the ceiling + the height of the ceiling joist. Cut vertically down the corner seams to that 2" location, then bend the excess OVER the joist & down the back side 1/2". Sort of an inverted "J"... Attach the support box to the joists with zipscrews thru the horizontal flat on TOP of them. This gives a pretty secure grip to support the weight of the Class A above it. Hopefully I said that in understandable terms...
cut the length of the box down, leaving the sides 2" longer than the thickness of the ceiling + the height of the ceiling joist. Cut vertically down the corner seams to that 2" location, then bend the excess OVER the joist & down the back side 1/2". Sort of an inverted "J"... Attach the support box to the joists with zipscrews thru the horizontal flat on TOP of them. This gives a pretty secure grip to support the weight of the Class A above it. Hopefully I said that in understandable terms...