The crown has a flat spot at bottom around 3/4" that gives you some wiggle-room for clearance, it doesn't necessarily have to all touch the box.
If you miter it correctly and use glue and brad nailer at bottom and from both angles through upper corners, then fill with filler or fill-stick, no backing is needed.
The also make a 2 part glue, one part is in a spray can, you have about 2 seconds to get the joint right and then it's "welded".
If you miter it correctly and use glue and brad nailer at bottom and from both angles through upper corners, then fill with filler or fill-stick, no backing is needed.
The also make a 2 part glue, one part is in a spray can, you have about 2 seconds to get the joint right and then it's "welded".
