Natural ways of enclosing a map?

for me it really depends on the whole theme of the map. if your map is themed around a forest you could always spend time detailing the outer sections of the map, and having a dense forest be a blockoff for the map. since you don't need to focus on the gameplay for the outside of a map, the best way to do it is to just think of the theme of the map, and just start detailing something that would work with that theme. trees, hills, and cliffs are great ways to "seal" off the edge of a map.

Usually I find that the most natural way to enclose a map would be to make a unpassable object around or by your map (a wall, fence, or building for instance) and just put a skybox texture over or around it. (I assume you know how skyboxes work)

Fence props are one other way of enclosing a map, and they're fairly universal, as in they fit with almost any theme. Depending on the type of map, you may be able to use large buildings to keep the player in, cliffsides, or a combination of both. If you feel your enclosure is looking too repetitive, you can break it up using roads blocked off by barriers, vehicles, or other props of your choosing.

People won't question a blockade to your escape of the playable area if it feels like something that would occur in an environment. For example, if you're creating a structure but don't want constant walls and closed doors everywhere, with the only open doorways being your exits to the structure, you can use something such as an open doorway filled by a dump truck or general human rubbish (just things left over because laziness). You also don't need to rely on cliffs, or other high structures everywhere to act as an enclosure - you can also utilise things such as deathpits to act as a 'this is the edge of the gameplay space' marker without physically being something big blocking your way. People also won't question low/loose blockades, like vehicles parked on a road that leads out of the map, or hazard tape stretching across a corridor you want to act as a wall