Made for a friend.
Unzip the folder C:\Program Files\Steam\steamapps\common\left 4 dead 2\left4dead2\materials\models\props_junk ( please create directories as they will not exist)
MAKE A BACKUP of C:\Program Files\Steam\steamapps\common\left 4 dead 2\left4dead2\pak01_dir.vpk
Open pak01_dir.vpk with a text editor ( back up this file incase you mess up ), I used Notepad ++
There is one edit to make for the skin.
Simply change "explosivebox001.vtf" to "axplosivebox001.vtf"
If you make more changes than that the game may not run so keep differences to a minimum.
- That's it, now instead of loading the original textures packed in the game's files, the game will load the ones you copied in the materials folder.