Another make-shift diffusion technique is to smear vaseline (petroleum) jelly on a plain (skylight or UV) filter. The more jelly you leave on the filter increases diffusion; the less you leave minimizes diffusion. Not putting any in the center will get you that "center spot" look.
I would like to foot-stomp an inssue raised earlier. SF filters are difficlut to judge by looking through them -- eitehr with the eye ball or with a "SLR". Often they look one way and the result on film is very different. I suppose that could be true also for non-film photography but I wouldn't know much about that.
I have an entire herd of SF filters. Of them all I use 1 or 2 and hate the rest... for a variety of reasons. The best way to know is to buy and try. The various filter makers show examples in tehir catalogues. That is the next best (and most affordable) way to evaluate. Look at both the Tiffen and B+W filter catalogues.