Counter using the countdown_timer_num_00 etc textures?
Haha I am not talking about art or puzzle quality, on that level the two games can not be compared. Portal 2 wins everytime hands down. But come on would it have been so hard to make an underground version of a prop_indicator_panel just as they did with the prop_button?
PortalStories.com
"Oh, in case you got covered in that repulsion gel, here's some advice the lab boys gave me: DO NOT get covered in the repulsion gel."
Yeah, they'd just need 10 texture indexes. 
[spoiler][SP] Alternate[/spoiler]
After having finished my cube counter using the countdown_timer textures I thought that perhaps others might be interested in using the vmf as a source for their own needs.
The chained trigger_once solution is quite easy to understand. The uppermost starts enabled, disables the "0" which shows the "1" behind, plays the sound and finally after a short delay enables the trigger_once below it etc.
Probably there are better solutions but this was the first idea I got and it worked and was rathe easy to setup.
Hope you might find it useful....