Today I completed one rack and got a good start on another. The completed rack is a single-battery rack to go in the engine compartment. Here it is after priming and painting:
And here it is with a battery snuggled down in it. The angles sticking out to either side will be drilled so that I can attach a hold-down (which will be attached to a plastic lid to keep fingers away from the 316-volt system):
I also got a start on the big 8-battery front rack. Note the scab plates in the middle of the left and right sides - I miscalculated by 1/2 an inch, which was more than enough to keep the batteries from fitting. After thinking about it for a bit, I just cut the left/right sides in half and butt-welded an angle bracket scab plate across. Should be plenty strong, especially given that the battery rack mounting points (4 of them) will be at about the CG of the cut sections.
This is the same picture, zoomed in so you can see where I plan on attaching the mounts, circled in red. I plan on using 1 1/2" box tubing (with 3/16" walls) for the vertical legs. Either 1 1/2 angle stock or box tubing will serve as the supporting crossmember.