The trick is to face the side gears first. They may apear to be flat, but they wont run true. then make your plate like the one in the picture. Mill the slot where the pin runs, then surface grind the backs of the plates, untill you have just 1-2 thou of clearance between the belly of the slot and the pin. If you have trouble finding springs, there is and old semi retired guy here in CHCH who made some for me. you will probably find that the plates you end up with are more rectangular than square. mine uses two springs side by side at each end, and one more just outside them. the more preload, the better. Then you can reduce the height of it enough to allow room for the shims which prevent the plates contacting with the idlers.