I've used a grade 8 bolt 1 inch in diameter for the pivot in the past and built the rocker out of mild steel with a double ball bearing center. Not that easy to make and mounting it is even harder because it takes some fabricating to bolt it to the carrier. I used 3/4 inch Heim joints for the locating arms.
If I ever did it again I would use urethane bushings for the pivot only because it is easier to fabricate and needs no maintenance. Additionally I would weld the pivot bracket to the axle housing/banjo if you are running a 9 inch instead of trying to bolt it on, it simplifies the construction immensely. Only 1/4 inch or thicker plate is strong enough to resist the forces this critical component will see without resorting to complex gusseting, it goes without saying that your welds must be done properly with proper penetration using the proper filler metal. If you are fabricating this to fit a cast center section axle like a 12 bolt then you must bolt on the pivot carrier.
How you construct it depends alot on how much room you have and your abilities/skills as a fabricator/welder. This is not a project for the amateur constructor mainly because failure in this area might cause a possibly fatal loss of control of the vehicle. If you do not have some sort of engineering experience or knowledge of forces I wouldn't attempt this type of fabrication, this goes for most anything suspension/steering or chassis related construction. Catastrophic failure must be avoided at all costs in this area with the preferred failure mode being bending not shear (except for the pivot bolt). Depending on your local vehicle laws this kind of fabrication might be illegal or at least forfeit liability in case of an accident (for example no welding is allowed on steering components here).
Be careful out there.
Outlawed tunes from outlawed pipes