I used 3" x 3" and it just fitted in snugly with a bit of movement for alignment. I took the same path as you with starting off inside the frame rails then working outwards from there. I made the centre section solid but made the outriggers of smaller material as a deliberate weakness. If I hit something square on it will do little damage, depending on impact, but if I hit something with the outer edge it will twist my chassis very easily as there a no diagonals to keep it square. Here is a picture of my bumper, the checker plate over the side sections is also a step to get up into the engine bay when on the side of the road.
You must be registered for see images attach