Here's a cheap trick for use with a single flash: back in the day before I had umbrellas, soft boxes, etc., I always carried in my camera bag, a largish, very white gentleman's handkerchief and a rubber band. With that, I could hand shape the cloth into a big hollow ball and rubber band it onto the front of the flash - getting a pretty good diffuser effect fast, for no $, and ultra lightweight. Setup time: fifteen seconds. Weight: almost zero. You'll be surprised how good a job this can do.