We're guessing that the reason the clamp won't hold the post is that those ears the clamp bolt goes through have bent in and meet before the clamp is tight. Enlarging the gap so this doesn't happen should fix it, except none of your photos show the ears actually touching.

But see how the sides of the slot aren't parallel? The holes through the ears are on the same angle...

If the ears don't touch, the clamp might have been prevented from clamping further by the angle of the holes changing so much that the clamp bolt prevents the ears coming any closer, because the angle increases as the clamp tightens. You should be able to check this by trying a thinner bolt that won't interfere with the holes, with some washers to transfer the force to the ears.

You might need to run a rat-tail file through the ears to allow room for the bolt in the clamped position... and then you may find that the ears now touch and you also need to enlarge the slot, filing the inside of the ears to make the slot parallel again.