Flimsy construction?

[Jeff]

The construction is sure unconventional and I'm curious. So opinions, thoughts, and critique are welcome. This is a basic rectangular box without dadoes and rabbets but it may have reasonable structural stability and won't be subjected to much racking. The "X" in the back is half-lapped in the center so they're like locked scissors. A plywood back (1/4"-1/2") will be added and fastened with screws/nails if necessary (probably?).

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[nn4jw]

The commercial metal shelving in my shop is constructed that way. The top, bottom and sides are bolted together. X braces in the back and movable shelves. No problems. It's a common type of shelf construction and as long as everything is attached securely there shouldn't be any issues beyond normal load limits for the materials.

 

[AllanD]

I can't tell how you plan on attaching the braces. If nails then I'm not sure if they will add all that much. It would be somewhat better to extend that cleat on top and the bottom board to cover over the ends of the braces.
If you are going to apply a plywood back then I really don't think you need those braces. Especially if you are considering using 1/2" ply. I would glue the back and use plenty of brads or staples. Particularly since you said it wouldn't be subjected to much racking forces.