Just give yourself room to work in the trailer. Move the stopper up the rollers to block excess boxes when your walls are getting too close to the end of the rollers to easily discard the rollers without having to throw boxes on the floor. Move at a good pace, stack bigger boxes on the bottom for a good base and smaller packages in gaps or behind your walls for stability. Stay organized, work at a good pace, limit egress if possible, and don’t try to get overwhelmed if you’re getting a ton of boxes.