Basement cinderblock wall tops - cover or fill
If it were my house, I'd get several bags of Quikcrete and take the time to carefully shovel it in there with a small garden hand-shovel and maybe some sand.  Quikcrete is cheap, reinforces your foundation from other problems, and fills the air gap.  No gap, no problems.  Then like you're doing now for rodent control and insulation.  Unfortunately, I can't back this solution up with experience.