I don't know how you'd use a light attached to your backpack as a headlight... but for my gravel/commuter/beater bike I bought a super bright small flashlight from Home Depot and attached it to my handlebars using a mount from a different headlight I had which just wasn't bright enough. I don't know if flashlight handlebar mounts are available anywhere, but you may be able to 3D print one if you know someone who's got a 3D printer.