Yea that did come up in my thought and i think that is what i am going to do, i was pondering the idea because i feel it won't give much downward support but it will definatly sturdy up the frame i think the corner posts will be strong enough to hold it up. I will add those too supports to the rear of the frame and how it works out.. thanksMy suggestion would be to build 45 degree angle braces for the back section. That should leave you enough space in the center for the depth you need, and still be strong enough.
i had some at home , u can find onlineWhere did you get the black plexi glass?