You don't want to use fiberglass batt for this. It's permeable and a chimney has a powerful draft effect even without a fire. It will still leak and will probably get wet and moldy.

You can always spot where the air leaks are in tops of walls up in attics because the fiberglass insulation gets filthy there. It's just an air filter in that case.

Chimney balloons are cheap and effective. Rigid foam insulation spray foamed into place is even better. Whatever it is should be damp proof and air impermeable.

For OP, there will still be enough air movement for the brick to dry itself above the plug if you do this fix.