Cut out a cardboard pattern leaving an 1/8" gap. Cut the fill piece out of 1/4" plate. It may need some beveling to get a flush fit. One of those red fabricating 90 deg. magnets spanning the flanges will hold the plate flush with the edges for tacking. Weld it in flush with the axle flanges. Grind for looks. No I didn't do this one.
Grain structure ? The grain direction is not a signifigant consideration in the application. Just don't weld the lock pin into the axle,and allow the part to cool in air. Do not quench in water.