I'm with them: don't do it. Bad idea. Bad for directional stability (rigs that come with one have been designed to account for it; yours wasn't). And illegal many places to drag firewood around -- there's a fine involved. Much cheaper to buy it locally.
That is a small trailer, lightweight, and it won't have a very high tongue weight to begin with. For every 100 lbs you load on the back bumper, you will probably take 80 lbs or so off the tongue weight. It won't take much at all back there to make the thing wag your tow vehicle like a dog's rear end. Very, very scary to drive, and extremely dangerous.