Let me see if I can find some.
First I lowered the tongue jack to the ground and put jack stands under the rear of the trailer. The jack up the front until you take some of the pressure off the frame. You want it and straight as possible. It will never be perfect because they were not built perfect. I was able to see the gaps inside my trailer close up by doing this. I let the trailer relax for a couple days.
I then added square tubing between the leaf spring hangers on all three axles and made the support brackets for between the wheel wells, lag bolted up into the walls then welded to the frame.
I have heard the story of the jig but to me there price was more than I wanted to pay. Mine has been perfect since my fix. You will get different opinions on fixes.