I could glue or otherwise affix a dowel to something flat, possibly.

I could buy a dowel from a hardware store.

I could use a pencil.

I could use a metal rod: this seems it would work better. Like, a coathanger may work.

I could possibly use the bolts to affix a bent coathanger to the bracket, and lash the shaft to the bolts to keep it in place. Winding a cable might tighten the lashing.

This seems unlikely to work given how my hands spasm and shake, but it's an idea and may work.