I am also in the planning stages of building my toter, the chassis is bought and here, and parts and plans are accumulating.
I found the top corners at a surplus yard just up the street from Bontrager's (can't remember the name, but it is just up the street), they had the cast aluminum corners for the front and back, as well as the extruded aluminum trim for the top, and lengths of rounded stainless to match for the front corners and top front. Basically will look like a nice flat roof race car trailer which will work good for me because it will match what I am towing. You're gonna need to plan a trip to Elkhart before you get too deep into the build, there are just some things you can't get anywhere else. For example that trim is in 20' lengths so you can't ship it and your local rv/trailer shop would rake you over the coals. There are a bunch of surplus stores there, plan on taking a big list and spending a full day or 2 there. I got a bundle of 10 new matching cargo compartment doors (framed with hardware, just screw them in) for $100 for all, and a nice new entry door with screen door for $90, you get the idea.
I, like most builders, I think tend to over-think, over-build, and over-engineer, and end up with too much weight and spend too much money. While it certainly can't hurt to build with bigger tubing with thicker walls, it is just overkill and weighs too much. Which you'll hate every time you fill up, and every time you get on a grade. Every extra pound takes power and fuel to lug around the country. My older Pace trailer (back when they were good, before they sold out) is built with the old fashioned hat posts in the walls and 1x1.5 square tubing in the roof and 2" angle iron for the top of the walls. The thing is 10+ years old, gets loaded to 20,000+ on a regular basis, totet all over the country, and still looks like new. And I guarantee that 40' trailer takes more flex than a 20' box on a class 8 frame. I've been trailer shopping the last few months and the hat posts are still common for the walls, and 1" tubing the first step up (which I don't care for) and 1x1.5 on the better trailers. So I'm thinking that 1.5" square in the lightest thickness you can find is more than adequate for strength, and is wide enough to make sheeting inside and outside easy. I do like the idea of heavier corners and perimeter, and Dragonslayer's idea of triple corners would make sheeting the inside simple, you'd have something to screw to on both walls which is something I wondered about. Actually, you could probably just use 2 posts there, so you could use rounded sheet metal on the outside for a cleaner look. Figure your side and end walls so the posts match up corner to corner and weld the seam all the way up. Anyway, don't over-build the size/thickness of your steel, you're getting your strength from the complete structure, not the individual tubing.
