Got to work in the late afternoon on the project and finished the left frame box by adding two horizontal supports and the 2x4 vertical post. This completed the left frame box, so I moved on to starting the right frame box.
Next I added three short horizontal frame stubs to support the left corner 2x4 vertical post.
Once the three horizontal supports were in place, I levelled the 2x4 post against them and secured them with screws. I also checked for level on three axes and made minor adjustments to keep everything true. I then proceeded with the next column of horizontal supports and added the next vertical 2x4 post.
Once that part was completed, the shelves slid in for a nearly perfect fit. Tomorrow I'll add two additional long horizontal supports in the right frame box to keep it sturdy, level and square at all corners. All told, I was pleased to have accomplished this much in only two hours.