You may not even have to damage a child trailer to get a perfectly useful cargo area out of it. I began dragging the bare metal bones of a trailer around during my lunch-break rides, figuring I'd get used to the drag and increased weight before trying to haul my dogs around in it, which was the ultimate purpose I wanted it for.
The mechanics had someone they knew with an unused children's trailer sitting in a garage somewhere, still in its box! They gave it to me, and it is even green, my favorite color. So I have a child trailer in mint condition, but no child.
Anyway, I love this thing, I use it for taking recycling to the grocery store, and once for a family bike ride. It is very easy to lay down the seatback by removing two screws with little plastic brackets. I can now slide the seatback off the top horizontal bar which holds it (and locks the sides of the trailer securely apart) and use the entire space inside the trailer.
I would like to add some sort of floor to mine. Perhaps a scrap of carpet would do the trick for dogs, but not any heavy cargo, think. I regularly see trailers on craigslist for fairly cheap. You might even find a bare-bones one that you can then build up how you like. The one I originally got was $5. I like my new one though, just in case I decide to have a family someday.
Good luck!