This needs explanation:
I used flat electrical panel plates to make the parts that support the wheel axles. 2 of the plates I took and inch off the top. The other two plates I drew a line across one (1) inch from the top. I placed these plates in the vise with the line even along the top of the front jaw, with a piece of 3/4" round tubing clamped behind it. Then I used a hammer to wrap the top of the plate back around the tubing. Those are like the one you see clamped around the tubing closest to you. The straight ones are clamped to the cargobox square tubing. At 1-5/8" in from the bottom edge, and centered side-to-side, I drilled a 3/8" hole, then used a hacksaw to cut a 3/8" slot from the edge to the hole. Then I placed a piece of 1/4" angle Aluminum (just a handly piece of scrap) to line all these up as I clamped them for welding. These aren't centered on the sides, but located just aft of the pivot point. Why? I don't know. Just because most of the cargotrike frames I have looked at, had it that way.
Here it is with the plates welded on to hold the axles.
Notice the 3/8" slot cut down to the drilled hole.
Here it is with 24" wheels
Looking at the bottom with 24" wheels on.
The rope is because I was trying drag it to the house to show my wife, and it just kept pivoting back and forth. So, I had to lock it down with the rope in order to tow it behind me. It will be okay once I weld the aft portion of the bike to the main tube and attach some kind of steering bar to the cargobox frame.