Small (7' x 4') early type railway container, used until the 1950s. A suitable load for medfits, highs, 3 and 4 plank open wagons.
Designed from the information and drawings on this website:
I wasn't entirely sure of the lid design, but having seen other photos of other containers where the sides / corners hold the lid in place, I have used that idea.
It needs lifting loops on the top of each of the metal side bars; they are probably best made from small chain links.
Definitely N, not 2mm finescale, it doesn't have that degree of accurate design.