While I don't think anything will provide the same look as a beauty dish, I think the next best thing would be a small softbox with the diffusers removed. Maybe you could even add a circle of cardboard in front of the flash tube in the softbox. This would make it pretty much like a beauty dish only rectangular instead of being round. Of course the catch lights in the eyes would give it away.
Just make sure if you add a cardboard circle, it is far enough from the flash tube so you don't start a fire. Maybe a circle of aluminum foil would be better.