Your concept is really good. Your veg side will work well with small ventilation.
When building your frame (as mentioned by Coho) install diagonal braces, or better yet put your reflective foam on the inside of the 2x2 frame, and sheet the outside in 1/4" ply (or thicker).
On your flower side I don't think two small exhaust fans will be enough for a 600W.
Quick point in ventilation; Its better to install an over sized exhaust fan and use a passive intake, with a circulating fan in the area, than expect good air flow through smaller intake and exhaust fan set-ups.
If you can, get at least a 4" inline fan, a 6 would be much better, and have it draw out of the box through your light hood, and discharge outside the box wherever if you're not concerned about odor. Make passive intakes down low where you have them on the back & or side.
Remember to be concerned about light leaks, while assembling (12hrs complete dark is very important). With sheeting on the back you can offset the penetrations on the ply & foam board to create a light trap.
Keep it simple and happy growing.