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Agree with 10box on what diffuser he's actually using, but to John's point there's plenty of cheaper ways to accomplish the same thing. I use stretcher bars, wood glue and Rosco 3008 with a flash head, reflector and grid.
If your trying to get a nice flat white edge in a reflection, I would use a flag to cut or block the light versus using the edge of your diffuser, if that makes sense. Then you don't need to worry about what the diffuser edge looks like. Or if its a square, I would honesty just photoshop it or get a soft box. I'm not sure having white tubes would actually help, would it? If the light is coming from behind the tube, it would be blocking the light on the product side anyways.