I decided to fabricate my own chassis as well. Knowing nothing about welding, I enrolled in a class at the local junior college.
It was 8 weeks for 3 hr every Saturday morning. I and a retired farmer were the old guys. Most of the other classmates were much younger folks trying to get qualifications for employment.
The shop had all kinds of interesting equipment. I really enjoyed the experience.
Afterwards, I had a 220V, 60 amp line installed in my garage and I bought a DC stick welder.
I did a few little projects before the trailer. I made a welder cart first. Then I made a steel post driver out of a bit of scrap pipe. I figured if the post driver did not fall apart, my welds were pretty good. It is subjected to a lot of stress when you pound into hard ground.

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Aside from the process of building the chassis, I wanted to learn more about metal working. I got a piece of railroad track for an anvil and made a charcoal forge driven by an old hair dryer. With that and the welder, I have made some custom hardware to attach to the trailer.
I used ideas from
https://www.tnttt.com/viewtopic.php?t=51991#p956707 to design my chassis. Since it was going to be light I chose 1 X 2 in tubes for the A frame portion.
Andrew's charts indicate it should be fine. The axle was bolted to a short piece of 2 in square tubing. The outer rectangular portion is 1 in angle. 1/8 in metal throughout. The finished trailer is 510 lb empty.

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The whole project was pretty slow for me but we got it on the road this past Fall and had a nice first trip.
Tom