Second trailer size. Definition. double or triple tow.

Sounds like a nice project, but you can buy any number of 7’W x 10’ L cargo trailers. Single or actually tandem axle, even on a short trailer. Height can be ordered taller.
There are many trailers out there that will fit your dimensional preferences very close.
Unless you’re hung up on the exactly 74” interior width which is silly. 7’ trailers are about 80” wide inside.

Again, great project, but not practical from a time and $ standpoint with so many mass produced options available that will fit the Bill.