There are a couple of things that bear mentioning with this dev files update:
1)The assets Ex3mx made for the main game are now available in the content browser, and you're welcome to use those.
2)I've come up with an extremely inelegant “solution” to the people-falling-out-of-the-map issue, in the form of a special object called “LevelBounds_BP”. You drag one of these into your level to define a boundary for that level, and if any player leaves that boundary, they're teleported back inside the map.
The basic “LevelBounds_BP” class just has a box which you'll need to scale up so it encompasses the whole level.
I've actually already added one of these to all of my levels and the copies of user made maps on my end. If you're thinking of updating one of your maps in future, or making a new map, it'd be good if you could add one of these to your level. It doesn't have to fit outside the level really tightly, but it's best not to leave a huge gap, or an escaped player might try and grapple back into the map and not leave the bounds.
Note that you can add additional collision shapes to the bounds, and there's also a “LevelBounds_Blank_BP” with no default collision box, so you can use other shapes if it makes more sense. To be honest, I've just used the smallest box the level will fit in so far though.
Anyway, like I say, this won't actually prevent players from being thrown outside the map, but it should move them promptly back in, since previously they'd have to quit and rejoin, or the host would have to restart the match.