When joining a lay-in section or fitting to a closure plate, a box connector, a non-lay-in cut-off fitting or any similar non-lay-in section or fitting 90 deg angle sealing plate is required at each junctionHoffman furnishes this plate with fittings
nVent HOFFMAN F88LSAG F10 90 deg Sealing Plate, 8 x 8 in, For Use With NEMA 12 Lay-In Wireway, Steel