Squareness is tricky in these trailers, most everything is curved except the subfloor. Heck, even the subfloor has some 1/8" high and low spots.
The best I could come up with was to level the trailer, then use a level for laying out the shelving strips. I also measured from the ground up inside of the trailer as an additional reference point.
Mine aren't perfectly level/square - but are probably with 1/8" true- which seems to be more accurate than anything else in most any trailer I've seen.
As far as the selves go, the right side of the closet is about 1/4" deeper than the left side. I just used the smallest depth for each shelf, that way I could shift each shelf either up or down a notch or two without recutting the shelf. The resulting fit is very tight and the shelves really need to be coaxed in place. They won't fall
out of place even on the bumpy forest roads we sometimes take.
I used 3/16" rivets ( 3 per track) and snap caps for looks. I figured the 1/8" rivets might not be up to the task of holding up the shelves when fully loaded.