Can't say I've seen a power strip "casing" but I haven't really looked either. Technically speaking, any components used for a power strip have to be designed to be used as such. It's not uncommon to see an ordinary junction box, cover, length of cord, and receptacle (or two) assembled as some kind of power strip. While it may be perfectly serviceable, technically it's a no-no because it's components are not designed to be used that way.

What are you trying to build?

What do you mean by power strip? If this is permanent wiring you have to use listed components and obey the codes. If you're making your own portable, cord connected device, not for sale, you can generally do what you want. Make sure any exposed metal is connected to the ground.

"While it may be perfectly serviceable, technically it's a no-no because it's components are not designed to be used that way. "

As long as it is cord and plug connected the NEC does not have a lot to say about it (or control over it).

Thanks everyone! I would like to build a power strip/surge protector for the home/office similar to what I could buy at a home goods store eg a power strip with 6-8 outlets.

I may not have used the right terminology. What I meant by casing is the box that contains all of the components. All of the ones I see at the store are made of a hard plastic. Does anyone know what kind of plastic that is? And is that my only option?

Look at DigiKey.

They have both metal and plastic chassis boxes of various sizes.

Thank you, Brickeyee, this is helpful.
Do you know what the rules are for using these chassis boxes? For example could I paint the chassis box and still have it obey the code? Or put my own plastic coating on the chassis box?

The NEC does o not cover cord and plug connected equipment.

NEMA covers it, but not tightly.