If you want to make a box I drew up some plans some time back for a box I made for my trailer.
(If I can find them I could email them to you if you are interested in building one. I meant to post them on my web site some time back but somehow forgot.)
It opens from the rear of the trailer and the lid runs forward on rollers. So openning and access can all be from the back of the trailer.
When slid into the closed position the front of the box locks itself down and the whole setup is waterproof.
Just need a catch on the back.
I built it from just over 2 sheets of 12mm 5 ply and coated it in epoxy prior to painting it. Will probably outlast my trailer.
It is around 8ft (2.5m)long 2ft10" (850mm) wide and 14" (350mm) high. Easily carries enough gear for 2 boats.
You can see the box in pics at the following URL
http://www.geocities.com/phillbrander/ttwp/tttrail.html