A selectively adjustable apparatus permits the holding down and stabilizing of cartons in a continuous motion packaging machine as grouped articles are inserted into the cartons. Spaced parallel hold-down rails (56, 62) engage the tops of cartons moving on a conveyor to hold them down and a side rail (97) engages the back ends of the cartons. A system of vertical and horizontal guide rails (38, 39, 46, 47, 57, 58), sliding guide blocks (41, 42, 48, 49, 63, 64), brackets (43, 44, 51, 52, 66, 67) and actuators (32, 73, 74) provide for adjustment of the hold-down rails to accommodate cartons of varying sizes.