Sri, I don't think that you need the rabbets or extra internal panel, it just makes it easier to assemble. I do think that you should add blocking at the new 90 deg. butt joints, and I would cut 2x material into 1.5"x1.5" triangles, they don't have to fit exactly at the mitered and doweled joints. I would ask GM what glue to use and how to reinforce the butt joints

I will try and finish up the plans this weekend and get a cut list for both 5x5 and 4x8. I am having difficulty coming up with a port layout that is both accessible, modifiable, and nice looking.