There’s a simple LED controller with an RF keypad that sells for around $2 postpaid from overseas vendors that you could wire in series between the batteries and light that would not only allow you to turn on the light but dim it if desired. I used these for my DIY bike lights and even for controlling LED house lights as a simple dimmer. Working voltage is 5V to 24V, 12A maximum current.
with CCTV power cord ends $2.01 postpaid: https://www.ebay.com/itm/RF-Wire-11Key-Remote-Controller-Mini-Dimmer-Single-Color-LED-Strip-XG/272593311472?hash=item3f77d3d6f0:g:mewAAOSwu4BVpmd5
or with bare wire ends $2 postpaid: H1 RF Wireless Controller Mini Dimmer for LED Single Color Strip Light for sale online | eBay You could attach a lanyard to the controller and hang it around your neck for easy access.