Im building a cage right now for a 8x5 dual axle. Im using 30x30x1.6mm painted RHS and 50x75x4mm gal mesh and its very tough. Not sure what guages you want to go for but this is plenty tough.
I agree not to place a cross bar at the rear end as you'll be cursing when you bang your head a million times on it! Ive made up a detachable roof rack which bolts it all together for heavier loads.
Ill post some pics once ive welded the mesh on.
Live life to the fullest, you have to go big and do everything with your all or why do it at all?