My 12000# trailer had a box built into the tongue by a previous owner. They welded a piece of c channel across the forward area of the A frame, then welded a plate across the bottom (supported by the frame, the c channel piece, and the front cross piece of the trailer bed). Then they cut a piece of diamond plate to fit over it all (frame edge to edge) as a lid. They used a piece of rod and bent it into a U and welded that to the inside of the tongue and cut a slot into the lid for a hasp. The handle is simply two pieces of angle iron welded back to back. Very simple, holds chain, straps, binders & blocks. And I don't care about water proofness -- it all drains out and is covered. Plus, it is as low as the trailer frame (nothing to trip over when crossing over the tongue) and will support a bucket -- and I can still open the tailgate (if the ground is flat so it doesn't hit the jack). I think it was likely made from scrap.
I would prefer a steel truck tool box if I was going that way. It can take more abuse and is easier to modify (I have seen a friends aluminum box on his atv trailer -- not too sturdy when it is out where it is exposed to full access abuse).
