How to get the yellow key in level 8, Final Outpost:

(Note: For a video showing how to acquire the yellow key on
this level, click here.)

The yellow key sits on a short pedestal in a small room.
Inside the room, to either side of the door, are switches
in the corners that raise and lower a cage that surrounds
the key.
If the cage is down around the key, raise it.

From inside the room, open the door and look straight down
the short hallway ahead.  About halfway down that passage
you'll see a brightly lit area.  Step into that lit area
and look to the right-hand wall for a square panel that has
a large X-shaped mark on it.
Activate this panel like a switch, and the pedestal where
the yellow key sits will drop down, allowing access.