General pointers for a CNC router

[JAZZCNC]

Interesting point. It seems to work well for Toomast but he did use 100mm box for the frame.

Possibly a tweak on this design could reduced the rail height and slightly increase the gantry height, that should potentially reduce any lateral racking.

I don't fancy bracing the front of the machine as it would make loading awkward.

Don't over think it because it's not required for the use your wanting it for, infact, for a sheet material machine you'd be better without the high sides to make loading and access easy, just build a strong gantry with stiff sides will easily do what you need. Over engineering might appear good at first until the real world issues stick there heads up because Accessing cut parts on a 8x4 with high sides is a pain in the arse unless your 7ft tall.!