We built the battery box out of 1/16" sheet steel.  The front, bottom, and back sides were made out of one piece and bent on the bench vise.  The two sides were then welded on, with a hole in the bottom for drainage.  We also welded on a "U" shaped bungee cord retainer:
Here is a picture of the battery box welded into position, behind the forward frame and in front of the engine mount plate.  If you look close, you can see the holes for the solenoid on the near side, and the ground screw hole on the far side.  You can also see both bungee cord retainers, one on the box and one on the inside piece of the forward frame:
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