I'm trying to figure out how the center bracket (L shaped) on backrest mounts to pack...HD instructions online are only for king size pack and chopped is only pre-drilled for center pad. Does it mount to the inside or outside of lid? Trying to find out as I don't want to drill in wrong spot and ruin pack lid. Thanks for any help! (2015 FLHX, with 3 bolt backrest being used)