OFF Topic, plywood Boxes?

I sold my hybrid, so it's time to build a camp trailer. I am unable to afford a cargo trailer at this time. I do have a nice 6 x 12 landscape open trailer. We have a few things ordered to do tent camping. After having a hybrid travel trailer, we hope tent camping is not to much for us? Thus I wish to build a few plywood boxes that will serve as storage on the open trailer. I also have a small fridge, micro, and sink I plan include on the trailer!I am looking for any type of help on building these plywood boxes. Mostly on how to calk, trim and paint so as thay can with stand the elements for a few years, or untill I can make a cargo trailer purchase. I thank you for any box construction tips. And I realize I'm off topic here, but thanks again for any help! Tomm