IDK about NY, but nearby states should have trailer builders. I had my cargo trailer built. They literally build the whole thing on site. They have standard axles piled up, and stacks of steel and build the frames right there. There is nothing about the frames except the tongue hitch that is pre-fab.
You should be able to ask them to build anything, any size, and width, any height that you want. The only odd part will be how the tires will fit as axles come in specific widths. Altering from their standard trailer frame sizes should not be a big deal, it is the same amount of work to cut the steel and weld it up.
Make sure they do not make floor joist 2 feet apart and try to correct it with thick plywood. Go for standard 16 or 18 spacing.
Have square tubes used, not the "hat" shape metal.
Decide where windows will go and have them weld the frame sized for the windows, and give them the windows so they have them there as reference when building it. Much easier to do this than to go back and cut and weld in scraps.
IF you plan on a roof AC unit, then decide where that will go so the roof can be built to support it better at that spot.
Dont use a roof AC, use a mini-split.