I’m pretty sure my road base was a home build job by a previous owner.
It is an A-frame using box section mild steel for the long sections (about 15x25mm section), plus a channel-section as an axle, to which the suspension units are bolted.
All in mild steel which is a pain as the launching trolley scrapes the paint off.
It is currently a lovely orange colour as that was the tin of paint I had to hand.
The launch trolley itself is 1″ galvanised tubing. I added a second axle to it, stolen from a kite buggy, so that I could recover singlehanded using the car and a long rope.
If I were making a trolley or trailer from scratch, I would look to buy/acquire galvanised tubes or sections, and bolt them together.