I just did this to extend a 10 ft trailer to 12 ft. I used leftover 2x6 deck wood and deck screws. It was easy. It took about 4 hours and most of that was measuring.
"There is NOTHING--absolute nothing--half so much worth doing as simply messing about in boats. Look here! If you've really nothing else on hand this morning, supposing we drop down the river together, and have a long day of it?" -Wind in the Willows