Once you figure out what batteries you will use in your car, you can make dummy batteries out of wood or cardboard to test fit. Then you can figure out where to put them, and build frames or boxes to hold them.

My car uses 44 total CALB 180Ah batteries. The original plan was to put 20 cells in the front and 24 in the rear, but I've since changed that to have 23 in front and 21 in the rear due to some modifications I had to make for suspension geometry.
Note the dummy battery used to test fit in the front photo.