I created a plane, textured it and positioned it. The repeated it for the remaining 4 sides. (I'm only using front, back, left, right and top.)
And since I'm feeling nice, here's the AGK2 code I use to create a sky in AGK.
Call MakeSky( Scale# ) to create and texture the sky, then put UpdateSky( Scale# ) just before the Sync() command. (Code assumes you have sky textures in the media/skys folder, in .png format.
Function MakeSky( Scale# )
Dim Sky[ 5 ]
For ObjID = 1 to 5
Img = LoadImage( "skys/" + str( ObjID ) + ".png" ) : SetImageWrapU( Img, 1 ) : SetImageWrapV( Img, 1 )
Obj = CreateObjectPlane( Scale# + 5.0, Scale# + 5.0 )
SetObjectImage( Obj, Img, 0 )
SetObjectLightMode( Obj, 0 )
Sky[ ObjID ] = Obj
Next ObjID
EndFunction
Function UpdateSky( Scale# )
YPos# = 0.0 - ( Scale# / 8.0 )
SetObjectPosition( Sky[ 1 ], GetCameraX( 1 ) + ( Scale# / 2.0 ), YPos#, GetCameraZ( 1 ) )
SetObjectPosition( Sky[ 2 ], GetCameraX( 1 ), YPos#, GetCameraZ( 1 ) - ( Scale# / 2.0 ) )
SetObjectPosition( Sky[ 3 ], GetCameraX( 1 ) - ( Scale# / 2.0 ), YPos#, GetCameraZ( 1 ) )
SetObjectPosition( Sky[ 4 ], GetCameraX( 1 ), YPos#, GetCameraZ( 1 ) + ( Scale# / 2.0 ) )
SetObjectPosition( Sky[ 5 ], GetCameraX( 1 ), YPos# + ( Scale# / 2.0 ), GetCameraZ( 1 ) )
For Obj = 1 To 4
SetObjectLookAt( Sky[ Obj ], GetCameraX( 1 ), YPos#, GetCameraZ( 1 ), 0.0 )
Next Obj
SetObjectLookAt( Sky[ 5 ], GetCameraX( 1 ), YPos#, GetCameraZ( 1 ), -90.0 )
EndFunction
Just paste the code in your program, doesn't require any additional initialisation, other than setting Scale# to something before calling MakeSky. (Or you could replace it with a literal.)