Ahhh okay. Thank you for clearing that up for me. I knew I must of missed something. That will be an awesome storage area!
Now that you've said you got him from a 10 year old girl, it all makes more sense. I'm understanding the scenario more now. That's good of you to notice that he apparently has issues with wire cages and getting his feet stuck, some piggies are just special that way and get their way into trouble no matter what. lol
I do agree that C&C cages may not look the nicest, so I can understand wanting to build something nicer looking.
As for kitties, as long as you have a lid, they won't be a bother really. Except for the fact they like to lay on the cages, which I'm sure as a prey animal, guineas don't appreciate having a large predator above them. And it's not like cats will listen to staying off
Good luck with your cage build!
As for some things to put in it, I personally like having bent grid tunnels covered in fleece, fleece tunnels (good for playing and sleeping), and plan to have some bricks with a piece of slate over it, with like a slate ramp, all aquarium siliconed together (where they can't eat it of course), unless someone says something bad about it to help file down their nails naturally and provide a cool spot to lay.
I also love fleece forests, as they are fun and take zero cage space.