I know this is tacky, but I would prefer to use a Cooler than a sheetmetal or fiberboard camp trunk.

Lighter, waterproof, and usually much cheaper. Plus, it doesn't stick out as much.

Can't secure it very well. TSA won't let you lock it, but if you use a nylon belt with a quick latch through the handles, it won't come open.

I used to haul around my rock climbing equipment this way. Works well and no-one every gave it a second glance.