Art. 370-23 of the 1999 Code does not address the use of screws. Art. 370-23(b)(1) addresses the use of nails, and "they shall pass through the interior within 1/4 inch of the back or the ends of the enclosure." However, I would prefer screws as they would not pull out as easily as nails. IF one considers the LB as an enclosure, then par. (e) would not allow screws or nails, but requires support within 3 feet of the enclosure. I must assume the Code means a strap support, and that would be my preference. In any case, the local AHJ has the final say.