Re: Anybody had a trailer built for to your specs?
I had my trailer built for me. It's a Pace 6x12 tandem axle, no side door, no vents, 3 ceiling lights across the top, 3 rows of e-track recessed into the walls, barn doors each with their own latch, and 12" of extra height (I'm 6'4"). I have to duck a tiny bit going in the door, but that's not a big deal.
The extra foot of height brings the walls to 6', and the roof is tall enough for me to stand up straight anywhere inside. I wanted just enough height to stand up in, but not so much it would add too much drag.
The interior lights are 2' from the front and back, and one in the center. I wanted to be able to see all the way up front when the trailer is empty, and all the way in the back when it's loaded full. It works well. You may not like the cheap interior lights, but you could always easily upgrade them if the wiring were there.
I use a 6' aluminum ramp that's heavy as hell, but doesn't bend even under the heaviest loads. For some things I like ramp doors, but I love not having one on this trailer. It's great to be able to back up to a doorway (or my front porch stairs), and put the ramp right inside. Also, the ramp is useful for other things, because you can move it around.
I had the e-track installed at the factory because it wasn't that much more money than doing it myself, and they recess it into the plywood walls. It looks and works nice. I didn't want a roof or side vent due to leakage issues, and there's never been a bit of water in there. I didn't want a side door in part because you can't (or shouldn't) put e-track on them, but mostly because there's usually a small step down just inside the doorway, and I didn't want anything that casters could fall into. There are times it would be nice to have a side door if I needed to get something from the front of the trailer, but I still prefer not having it.
If you're leaving it outside, definitely don't have it painted black. White is always a good trailer color. Also, get a 3/4" floor. Some lighter duty trailers have 5/8" or 1/2", but with heavy sound equipment you'll want a heavy floor. Also, you might want heavier axles. With the trailer weight and a normal amount of gear you can get up to 7,000lbs pretty quick.
FWIW, my 6' wide trailer is about 68 inches wide inside. What that means is that three 22.5" wide road cases fit perfectly across the inside of my trailer, which makes it really easy to pack. That may not be a consideration for you, after all, it wasn't a consideration for me when I ordered it, but it happened to work out really nicely. If I were ordering another trailer I would definitely keep it in mind.
Anyway, good luck with your trailer ordering. It's nice having something built exactly the way you want it.