Photography Bounce Light

How to make a homemade diffuser?
I usually shoot concert photography with no flash, but I am shooting a concert this weekend and there will be very little light available. I am using my external flash. I want to be able to use the flash to light up the bands, but not have the photos washed out. What could I make a good diffuser out of? I was also wondering if I could point my flash up and use bounce light, but the ceilings are high. Does anyone know how high the ceiling has to be before bounce flash will not work?
A coffee filter or tissue paper (like the gift bag stuffing) works well for this.
Single Light Studio Portrait Photography : Bounced Light in Single Light Studio Photography