I have built MANY MANY open, and enclosed trailers. I worked for a local fab shop, and I was 18 years old. I built a lot of them from 18 footers, to 16 footers, to a 32 foot tandem axle goose neck with 2 25000 lb dualed up axles. Its not hard, just make it square. Measure 3x, then measure again.

Kasey