This classic corruption of technological eras is often seen in adventuring bands. It has many forms and uses, just as real flashlights do. As soon as a party gets access to a continual light spell, you can usually bet on this item being crafted.
The basic prinicipal is to place a continual light spell in a tube in such a way as to limit it’s ability to shine light out except out the small front opening. The DM’s perticulars about spell targeting and enchantment make some basic variations to the item.
For example, some DM’s say that enchanted items consume the material at a rate dependant on the hardness of it, so scrolls have a short shelf life and spell books are very valuable, but enchanted swords are not uncommon. In this case, cast continual light on the smallest piece of currency your have or on an arrowhead, and insert that into the map or scroll case or hollowed out horn. If the DM allows more refined spell targeting, simply cast continual light on the bottom of the tube facing outward.
Flashlights can be used as light sources, obviously, but also as signaling devices, alarms, and handy distractions.
Continual Light - eternal torch light brightness in a 60 foot radius.