NIC 77 stated:
I'm about to start a 4 x 8 cutting area shopdroids build myself. It's going to be for Routing and Plasma. The one weakness, in my opinion, of using the shopdroids design for larger builds is the lack of stiffness. An aluminum T-slot extrusion 2 inches by 2 inches could have alot of flex over a 4 foot span.
NIC 77, I am builting a Shop Droid for routing that is 83-inches by 53-inches. I use 3030 t-slot from 8020. ShopDroid [custom sized]
I did moment of inertia calculations on the 83-inch length and with the use of a leg in the middle got the deflection to 3/10,000-inch assuming a load of 500-pounds evenly distributed, and support only at both ends and in the middle, and this did not take into consideration any of the cross beams which reduces deflection even more.
Though I do not have the statistics for the material you are going to use, my suggestion to reduce deflection to next to nothing would be to put a leg in the middle of both the sides. The deflection over two-feet will be almost nil. The deflection over the 8-foot side will be almost nil as well.