One thing I do, esp on an expensive bike is take a piece of thin luan plywood ( I have a bunch laid up from old woodworking projects) the same dimension of the bottom of the box and screw 2x4 blocks to it, the same spacing as the forks and rear triangle and the same wheelbase as the bike. I'll use wood screws and washers to attach fork and rear dropouts between blocks. This totally immobilizes the frameset and prevents damage to the dropouts on both ends. It adds very little weight, and maybe its overkill, but I've shipped over 50 bikes and never had one arrive damaged.