I've got an awesome local mom amd pop store that is stocked with 3, 100' long isles full of Hillman hardware boxes full of every fastener available. It's mamoth.
For the cart I used a flat headed anchor pin that had pre-drilled holes along it's length to accept cotter pins.
I couldn't find the dia. I needed so I took the size smaller they had and used an M6 washer to shim and make the head large enough and then lined up the closest hole for a cotter pin on the other side.
If your hdwre store stocks the Hillman line ( a Cincinnati based worldwide fastener supplier btw), you should find something a little more robust than a cotter pin.
Hell, even a 3" length of round steel rod stock or small dia. re-bar cutoffs from a concrete job would be fine and far beefier than that cotter pin.

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