I spent the evening playing with Google Sketchup, a free 3D "CAD-lite" program. It has a $450 professional edition as well, but I did not get that - just the free version. To learn it, I designed my rear battery rack. The whole thing will be made out of 1 1/2" angle iron, 3/16" thick. Here is the top view, with the top dimensions:
Here is the bottom view:
Here is from the right side:
Here is an iso view:
Here is a perspective iso render, with texture and shadows:
And here is a render with the 9 batteries installed:
Sketchup seems more than adequate for simple drawings like this. I would not design anything more major than these components with it, though.
One other detail - although this set of drawings shows just the rack, the entire box will be enclosed (since it fills a hole to be cut into the cargo area). I plan on using relatively thin steel and welding it to the outside of the rack. Proably 20 gauge or so.