I respectfully agree with Wagne223, it will probably be easier to practice than to carry around an extension you have to hitch/unhitch.
I have a short (and narrow!) trailer that I can't even see in my side view mirrors, and my truck has a cap on it. The only way to see it when I'm backing is when it starts to turn and pops out (in the rear view mirrors) on one side or the other. I can still back it, it just takes practice, and going slow.
Hint: First practice with a small empty trailer you can move by hand, or otherwise imagine you are pushing on the trailer tongue hitch by hand. That is, imagine you are the hitch ball. Take note which way you push the trailer tongue to get the trailer to turn in desired direction. Take note how if you continue in this direction trailer will jackknife. Take note how once back of trailer is headed in intended direction you must change the direction you're pushing (i.e. follow it around and push straight) or trailer will jackknife.
i.e.: "Be the ball!"
THINK in terms of what the ball/trailer tongue is doing (or needs to do) in relation to the trailer.
Once mastered, now do the same when it's attached to vehicle, that is, think in terms of steering the ball on your bumper to steer the tongue of the trailer, just like if you were pushing it by hand.
:2cents: