Okay, I want to delve into some crazy talk for a minute. Has anyone tried using strut channel for building a chassis? See it here:
http://www.mcmaster.com/#strut-channel-systems/=by28wp
Yes, it would have its limitations, but there are some great connecting plates and angles available for reinforcing the frame. If you used the slotted (or unstotted), deep channel struts with connecting plates (style 4 and 5), would that function well for a light-duty chassis? There are even some telescoping channel and hinges available. Its not the cheapest stuff in the world, but it is not that expensive either.
Like I said: crazy talk. But, man, it would be fast and easy to mount a trailer to.