Skiers and snowboarders use toe warmers. I imagine they'd be perfect for your needs. You open them, peel and stick them on your toes (over your socks), and then put your shoes on over them.
They're not rechargeable but they're cheap enough - less than a buck a pair. I get them by the case at Costco every fall but here they are on Amazon. https://www.amazon.com/dp/B002PDBUOU/ref=cm_sw_r_cp_api_.GopybPT3W9QN
Might want to consider the small hand/foot warmers tossed into a small zipper pouch with the meter.
https://www.amazon.com/Grabber-Toe-Warmers-40-pairs/dp/B002PDBUOU