I used this dealer before and am very happy with my cargo trailer that they built to my specs.
https://cargooutlet.com/
They represent three different manufacturers. The maker of my trailer was Diamond Cargo Trailers, located in Douglas, Georgia. That area of the state has multiple trailer makers. From what I understand, you must work through a dealer. The manufacturers do not sell direct to the public.
Some of the customizations can be done on the website. Talking to the dealer, I had more customizations made. For example, my trailer has a V-nose and a rear-ramp door. I learned from the dealer that I could have a walk-door installed in the Vee, instead of the on any of the straight sides. I transport tall furniture items. This gives me more wall space so I can transport more items strapped to the walls. They did this for no extra cost as well.
I also picked up my trailer at the factory. It was convenient and saved me some money as well.