I did go out and buy an underbody storage box, which is weather sealed and lockable made of sheet steel and nicely powder coated. Its a serious box for a serious price. (I swindled the store down to $250 from $400 because it was scratched. Its made from a company called Buyers. But as for making your own I decided early on to just cut the entire bottom of the sides of the bus off, and build new sides with all the storage I could imagine. In this process i am also lowering the sides to about a foot from the ground. Now I am just going to build new compartments from 1" by 1" tubing as a frame, then weld sheet metal in to seal them, finally I am re-skinning the whole thing and building my own doors. I am also going to use heavy duty drawer slides from Lee-Valley, http://www.leevalley.com/
, so that if I need to do some maintenance on the equipment it is easily accessible. The other thing to keep in mind is making your doors seal right. I ran into problems with this when I built a tool trailer a few years ago. I finally decided that the best way to keep the water out of my trailer was to build a lip that stood out from the body of the trailer around the openings, then I built the doors with another lip around their
edges. So when you finally install your doors they completely cover the opening and the lip on the door caps the lip on the body. I had this trailer driving through pouring rain and Canadian snow storms and I have never seen a drop of water get in. Lee-Valley also has hardware for pretty much anything you could imagine. I know they are predominantly a Canadian addiction...or store I mean, but I think our neighbors down south have a few stores and you can buy on the net. A warning though, I have rarely gone on their sight and not spent hundreds of dollars so beware!