There's an explosive device that uses a timer for the propane-tank and oxygen-tank for l4d1 and 2, and it works the same way as shown in the video for the Explosive ammo box.
Link here:
https://forums.alliedmods.net/showthread.php?p=1442706
If any one does find the link to that explosive ammo plugin as well, I'd be interested in that one too, especially with its beacon effect when the bomb is in its countdown mode.