The traditional method would be to vignette the enlarger during printing. Since dark becomes light in that process, the result is a white border in the print. You can adjust the hardness or softness of this border by varying distance (or by moving it during the process, as one might for dodging). An article on this from Shutterbug mentions creating dark vignetting in the print by doing a double exposure, with the center masked out in the first.

But since you're shooting direct positive (a.k.a. Polaroid) film, that's out.

A center ND filter (as opposed to a center spot filter, which as you note are used for a blurring effect) might work, but these are really designed to compensate for the vignetting of wide and ultra-wide angle lenses, so they tend to be only a couple of stops. That might be sufficient for the effect you want. (Do note that you'll need to be stopped down quite a bit for it to work.)

However, "not cheap" is an understatement for some of those medium-format filters. You could try getting a budget UV or skylight filter and using Vitrea glass paint to make your own. (I have not done this, but I'd love to see the results of someone trying.)