Here is a link to a nice little set up. http://www.lawnsite.com/showthread.php?p=3290687#post3290687
sorry I don't have pics of mine handy they are here somewhere. Mine is 8.5 wide anyway.
Keep long handled tools rakes, shovels etc. on hangers so you can reach them without climbing in and out.
Hang and secure tools, all tools.
Get a nurse tank, mount it high on the tongue and keep the 2-cycle and other engine fluids in a cabinet below the nurse tank.
Put racks on the roof for ladders.
Keep the trailer locked when ever you are not in it. combination lock for the back door works well on job sites, quick simple. Use a better lock for the evening.
I would use as little wood as possible screw,weld, bolt whatever you can. Wooden stuff always comes apart eventually.