I decided to take AdamDZ's magnificent cart design, and adapt it to my gutted Instep trailer. The square shape of the frame beams makes the In-Step very easy to work with. I used angle irons to attach the box to the frame. The holes in the angle irons give me the option of moving the box further backward later on if needed. The way that the bolts attach to the recessed areas inside the box should keep items from being scratched by the button heads.
I still need to finish trimming the angle irons down to size, but that's merely a cosmetic thing. Reflectors and other accessories will be added on later.
I also bought cross braces too attach to the angle irons, underneath the box, but I'm not sure if they're really needed. The setup feels pretty sturdy the way it is.
I'll post more as the design evolves.