Anything can block off your map, as long as it feels natural (in other words, players won't think they can get past it).
And sometimes all that takes is a knee-high guard rail.
Funny enough, just like in Hydro itself
I don't think using an official map's skybox is stealing. You guys all use the 2d skyboxes too ;P
Why don't you make your own custom 2d skybox? THIEVES! XD
Plenty of us actually do use custom 2d skyboxes .
Because Valve supply us with the textures and openly invite us to use them. >_>