Yesterday and today I spent some time putting my very poor woodworking skills to the test. I have mocked up the battery holders for my battery pack to confirm my design and to check I can hold 120 cells in this setup.
The setup has three (3) plates where the final will have four. Two outer ones form the end of the double width portion of the batteries (80 cells in total) with holes in the inner two to support these batteries. The inner plates will also terminate the single width of cells that are below the frame and above the motor.
Maybe a video will help.
RG250 Electric Conversion Blog 