First......do you have a sissy bar attached or just the horizontal rack? I would think you could find something that would bolt, strap or possibly put on a quick release attachment to hold a trunk. Second.....how big of a trunk are you thinking about? I've used a Pelican 1520 series that is a weather-proofed case before with just bungi cords holding it. Works great when you need that extra amount of storage. I'll start looking around.