First off I won't have much of a budget.
It would be a cargo trailer.
No sides only a back and bottom.
I would use a tarp in case of rain.
I am pretty good with my hands.
My preferred building material is wood but
I used to bend electrical conduit.
If I can find a pipe bender pipe is fairly inexpensive.
To me the hardest parts to build would be
the wheel brackets and the bike attachment.
If I knew how to post pictures here
I could show you one that suits me perfectly,