I'd use the custom white balance setting. Grab a white balance gray card, or just a white piece of printer paper
Whatever you do, don't use plain printer paper. Such paper has fluorescent optical brightening agents in it and is actually blue. Some can be strikingly blue. Because the color comes from UV fluorescence, the color of the paper changes depending on the UV content of the light. And the paper substrate (that they then saturate with fluorescent dyes) is usually rather yellow.
Styrofoam coffee cups are actually superlative white balance tools. They have a flat spectral reflection (the only thing that matters) of about 80% (which is good for this sort of thing). And, their curved shape lets you sample all the lighting in the scene in one shot. Such a cup will probably be too small for an in-camera custom white balance, but a lot of packaging material is made from the same stuff, and that'd work well. Use the sheet for in-camera white balance, but take a shot of the cup for later use in RAW processing (and eyedropper from different parts of the cup to see the range of light source colors in the scene).
If you have any sheets of high quality matte photo paper, that's a good option, too. Canon Fine Art Watercolor, Red River Aurora Natural, and Hanhemule Photo Rag are all excellent choices. It might be a bit too reflective for your stage, but Tyvek (that un-tearable stuff that envelopes are often made out of) is another option, and the back side of Canon's large format Tyvek Banner media is uncoated Tyvek. Tyvek actually has about 98% reflectivity, and is only bested by (pure) Teflon. But I'm guessing you don't want to spend as much on a 24" Spectralon target as you would on a 1-series body....