L4D Ambient Generic Addon directory structure

[tron00]

I had sound working perfectly but did not do it cleanly. I modified my L4D\scripts\game_sounds_ambient_generic.txt to include my level file IAL_sounds.txt. I had the WAVs in the L4D\Sound\Ambient\IAL_Custom folder. So, the DLC update comes along and rewrites the game_sounds_ambient_generic.txt file. Now I have no sound. What's the RIGHT way to do it using the Addons\Custom_Mapname format? I've decompiled some VPKs and see a lot of people messing with soundscapes but I'd like to use Ambients.
Thanks,
tron00