If it was me I would start by making a mockup out of wood and cardboard. Cut a floor of plywood, and add bulkheads of plywood or wooden boards to make a framework for the body.
Copy all dimensions off your trailer, but smaller.
Cut out pieces of cardboard and tack them to the frame. If it doesn't come out right, so what? Throw away the bad part and get some more cardboard.
When it is all done you can carefully take it apart and use the cardboard as patterns to cut out the aluminum sheets.
It would also be possible to make the framework out of steel tubing (copied off your wooden frame) and rivet on the aluminum sheets.
Copy the body off one of the old models made out of all flat panels such as the "whale tail" or"13 panel end cap" models before 1955
. Then you don't have to form any round shaped panels, they can all be cut out of flat sheets.
Probably the best way to make an opening lid, would be to have the end caps permanently fixed to the frame with an opening lid in between them.
For small wheels what about those dollies you can buy for moving furniture etc? I see them on sale for $39.99 or less and sometimes you can buy just the wheels at certain hardware and tool stores.
You would need to get a longer rod for the axle, or cut the original one and extend it.
How to make the windows, the easiest way would be to just paint them on.
For the trailer hitch, the old friction style sway eliminators had a small ball and socket joint, maybe you could use one of them.
It would be quite a project but not impossible if you were handy with tools.