What textures/props are these?
How do I make the grey circle-cutout piece and the large yellow serrated circle, etc. seen in these screenshots (from the Alive And Kicking Mod, BTW)?
http://media.moddb.com/images/mods/1/19/18283/sp_a52_chamb160007.jpg
http://media.moddb.com/images/mods/1/19/18283/sp_a52_chamb140003.jpg
I've looked through the texture and models and the only thing close is some of the destroyed wall props. Did they make these themselves, or am I searching for the wrong model?
I've seen them in the walls and roof of the walls in the destroyed theme,
Decompile the file sp_a2_laser_vs_turret and look around, there are the circles in there, near the turret quartet, I think.
Though, they're crooked and kind of yellow
Yeah, that's the problem. I found some models that look like them (wall_dest_), but they're all destroyed. I'm starting to think the AAK team made non-broken versions of those models, but I kinda hope not.
Have you checked the skins of the model?
How do I do that?
In the model viewer in hammer (the green model browser) find the wall with the hole and click on the skins tab
Um, that just shows what alternate skins it has (none, on those.) And I'm pretty sure you can't use model textures on brushes and vice versa.
Are they props or textures?
If you like them you can make your own custom textures in photoshop, port it over to vtfedit and just use the "clean" versions" of the textures in hammer( just learn to make a smooth circular hole in the middle first 
(though you don't have to cut one because the hole will be transparent like a nodraw) )
They are custom textures. Groxmapper made them before Portal 2 for a collab map, and he decided to use it again.
The more you give, the less appreciated it will be received.
Ok then. Thank you all anyway.