As mentioned in Step 4, I created a shelf extension piece. I used two barrel bolts to attach the two pieces of wood. In future I might use another type of latching system, a combination of bolt latches and draw latches. The problem with the barrel bolts is that the extension piece can just slide forward. However, I found that the pin that you grip to move the bolt just happened to be exactly the right size to fit between the ground and the shelf when the shelf was extended and this stopped the two pieces from sliding apart.
Note that I attached the barrel bolt latches so they face opposite directions. I'm not sure that this is necessary, but it is what I did. Remember the bolts will be on the bottom of the shelves, so face down the side of the wood you think looks most attractive when screwing the latches to the back.
Please comment if you improve upon this part of the project. I will reference you in future edits.