I've heard of using coffee filters for white balance, but will a Styrofoam cup let in enough light if placing over your lens?
I'd be rather leery of using a coffee filter.
The raw materials used for paper aren't spectrally flat. There are some high-end fine art papers that are quite good -- if it's from a reputable source, is expensive, is labeled as a natural white, and is advertised as free of optical whiteners / brighteners / etc.
, then it's probably pretty good. It even has a chance of being better than the typical expensive photographic white balance target. Even then, it's the coating, not the paper, that's white...so you'd only want to use it as a reflective white balance tool, not a transmissive one.
But a coffee filter? I'd be quite surprised if it's spectrally flat. Not saying it's impossible, just that that's not how I'd bet.
With a typical styrofoam cup, you'll get about the same meter reading as with evaluative metering. That is, put the cup over your lens, and whatever your in-camera meter reads is just about what you should be shooting at. I wouldn't rely upon it as a metering aid without doing some testing and experimentation -- and, obviously, strong light sources hitting the cup but not part of the scene (such as stray sunlight) will skew things significantly. But it does mean that you wind up with, essentially, typically, a full-frame neutral 18% gray card shot. And, while, on the one hand, you'd like it to be brighter to reduce noise...you're dealing with the whole frame and so it's trivial to average out the noise. Or, of course, you can adjust the exposure to get a brighter rendering...it's not like you have to worry about sharpness in an out-of-focus macro shot of the inside of a coffee cup you're just using to get a custom white balance....