How can I build a trailer tongue out of wood?

I am building a small (36“x18”) cart/trailer to be pulled behind a 11 horsepower garden tractor. All I have to work with is some 2’x4’ boards and a 11” long 4’x4’ piece leftover from a project. The problem that I have is that I need to build a tongue to be able to pull the cart/trailer. I do not have any metal AT ALL to use, so that is not an option. Can anyone give me an idea of how to design it?

(It will be carrying a few hay bails—the smaller rectangle size ones—so it doesnt need to be overly sturdy, just enough to carry about 100 pounds)