Miss my 'trunk'...

I guess I want my cake and eat it too.

When I had an aluminum road bike, I had a rack with a trunk on it. I loved it! I could stop at the produce stand and pick up fruit, at the Farmer's Market when on my weekend ride.

Now I have a carbon bike, and I love the lightness and speed, but wondering if there's a way to add a trunk for particular rides that might include a picnic or a 'fruit stop'.

Could it be as simple as changing out the carbon seatpost for an aluminum one, to use a seatpost clamp rack to put a trunk on it? Do I have any other options? I'm not in the position to buy another bike just yet.

Thanks!