Just make a room over your map. Works for me. lol
Never ever ever ever ever "box up" your map to get it to compile. This is the second worst mapping practice I know of. The worst being leaving it unsealed and releasing it without fully compiling it.
To make a sky, you actually have to create a sky brush (that is, a brush with the "sky" flag set). If you select one of the sky textures, it will have this flag set by default and you will not have to do anything beyond creating a brush with that texture.
To align barrels, simply create them on a 64 unit grid (start out with a 64 unit cube, then make->n-sided brush). Once you've created it, hit alt+L to lock the texture and you can move it wherever you want.