I'd never seen one of the pro-built ladders either, so I don't know that mine is how others have done it previously, but it's pretty basic.
The bottom two beams are welded to a piece of 2"x1/8" flat stock. It is through bolted to the bottom seam of the van. The upper bracket parts can be seen here:
It's basically a J shape made out of various scraps of 1/8" in steel. The inner clamp is thinner, maybe 3/32", steel that is bolted to the gutter hook and keeps the upper pieces from shifting.
I was up and down it several times yesterday loading boats, and it is rock solid. You wouldn't think the emt would be as sturdy, but it seems totally up to the task.
I'll try to post a couple more clamp pics tomorrow. You might be able to get some old Thule, or Yak "Q" type clips and mod those for attachments too.
Cheers,
Sam