I can tell you what not to store them in.....I had one of those plastic "egg crate" boxes in my side box to hold my hammer, nails, corner protectors. Got lazy one time and threw a couple of chains in it, the links vibrated through the holes in the sides of the crate. P I T A to get them out.
I measured everything between the frame rails and cross members and there was something on the frame, I dumped the air and measured to the driveline. I had a local fab shop brake the steel, it has a flat part that sets on the frame on each side, then it drops down. I welded ends in, the frame for the deck plate is made from 1" X 2" box steel, and there's flat steel the same thickness under the dead sections that have box welded to them and the aluminum bolted to that. The hinges on the lid are welded to the box flanges so if I pull the box it's all one unit. The steel is 1/8" which I think is 11 gauge and it has been plenty strong. Let me know I can scribble out a diagram or snap you some better pictures.
Gotcha. I believe i understand. I wasnt sure if the top flange was bent or angle iron. I see what your saying. Looks good and very handy for sure. I need your rear fenders too!
Now if you would only get them chain crates made out of something light you would save 100 or so pounds, lol!!