Hello Quidgiboo,
you did a nice job with this. I added the missing entries, so if you play the latest master you can now check out your work in game. Just choose the frozen_crash_medium_industrial or frozen_crash_medium_nature maps in the single player skirmish menu - or keep on shooting down Supply UFOs in campaign mode
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There are only few things I noticed that need some more work. First, all brushes need to have correct levelflags. Second, the main texture of the terrain (snow002) has a different scaling than the other tiles. Also I would like to have the border textured also, to stay consistent within the theme. Also there is a gap in the brushwork next to/below the broken right wing of the UFO.
Beside that I recommend to copy some trees from other tiles and place them on the map. This will help to break the lines between the different map tiles and give the assembly a more consistent look. Same goes for the snow particles. Another thing is the fire (misc_fire) that is burning out of the snow. I think you will notice this stuff yourself if you do some playtesting.
Just in case you want to dig even deeper:
There are two different battlescape models for the Supply UFO. First is without a roof (so one can look into) and is shown on level two only. Second is a complete model which is shown on level three and above. The inner part of the UFO is brushwork, which is also visible on level two only. As on your map one cannot walk/look into the UFO, you could remove the first model and the brushes.