A place in NC called Diamond Deluxe custom made the box - price was $860. They make animal transport boxes (dog boxes) and it was a modification of two of their dog box styes:
http://www.diamonddeluxe.com/r_style_dog_box.php
http://www.diamonddeluxe.com/yy_style_dog_box.php
The company was very easy to work with. We emailed a couple diagrams back and forth, they gave me a price and I said "go for it". I sent them a check for half-down and they started building it. They told me the domed roof was a little more expensive but much stronger than the flat roof, and after looking at the lid's underside I believe them. I really wanted the flared sides like the old Army Jeep trailers, and that's what they built for me. They also gave me a price for a non-flared box same height (straight sides) but with a domed lid that would perfectly fit a little HF 4' trailer that was around $650 if I remember correctly (but check with them, as how you spec it out will affect the price obviously). I'm very pleased with it. This thing is HUGE inside with the lid open. I have about 26-27" of usable height inside with the lid closed. The old Army Military trailers were only about 18" tall. It's all about usable cubic feet (length x width x height). Pretty much you mount this thing to your little HF trailer frame, bolt fenders to the side and you could be done.
If anyone else has a box built post a picture of it with your trailer - I'd be curious to see if anyone else goes this diamond plate aluminum box route. If you've ever priced aluminum truckbed tool boxes you'd know this was extremely reasonably priced.