You indicated a 6x12 build with a 6 foot height, but then mentioned that a company will not build a "custom height" for you.... what height are you hoping for ? In my case, I once thought I needed extra height, but it wasn't until I walked in an exact 6' high ambulance chassis that I realized that it works perfectly for me being 5' 11". After all, I am not pacing back and forth all day in a 6x12 space... You go in, sit down, lay down, whatever... I do my WALKING outside !
I have to say, I have been quite happy with the Amerilite ATTX 6x12x6 (
viewtopic.php?f=42&t=70777) I bought from S&B trailer sales just north of Green Bay, Wisconsin. The price was already less than other trailers I had been looking at, so it did not take much thinking to make the decision. I recall the fellow telling me the price was so competitive because they ordered a number of matching units in one order, getting a better purchase price. I also feel that the quality is really decent.... I learned too, to be wary about some of the aluminum trailers that show a lower empty weight than others because then it obviously has either thinner, or just less structure to accomplish that low weight number. Mine was a little higher than others I looked at, but I understand now after I have gutted and seen what it consists of. Very well made.
I thought I would go the custom order route initially, but frankly, the cost would have been so much more that it was not worth enough to me over just working with the options it had right off the lot. For example, it did not have an RV Door... Didn't matter in the end, because I was able to easily put in an RV latch, and the door is a pretty darn good door in the end. And, a "magic screen" setup seems to work perfectly well, and lighter too. I see you indicated barn doors. That is always a personal goal reasoning... I wanted Ramp, and that is what I got, but if it came with barn at a reasonable price, I could have worked with that too. Always a way if there is a will.
After years of steel frame utility trailers and watching them rust away, I really enjoyed the light weight of my first Aluma utility trailer, so I went aluminum when it came to my first personal cargo trailer. I've taken time to coat the nooks and crannies of the bottom with Fluid-Film, with a double extra soaking where they bolt the steel suspension parts to the aluminum frame. THAT is where corrosion will be the trickiest to control.
Lastly, with all of your typical 6x12's claiming no more than a 2990 Gross weight, I realized that I would need to go aluminum if I hoped to include as much as I have in my trailer.... If one intends to do a conversion on ANY 6x12, you find out that it is a challenge to keep the gross weight within spec.