I’ve got an old Nitecore SC4 and use that off unregulated 12V (e.g. voltages similar to what you’d get out the cigarette lighter output in a car.) Sadly 21700 batteries (unprotected) barely fit, but finding something that will run off unregulated 12V and fit long cells is proving difficult. Vapcell make a 12V model but it apparently shouldn’t be used on unregulated 12V.