you can try doing it at the printing stage using selective diffusion and heavily burning the edges to vignette, or stack some filters on the end of your camera lens and selectively grease one of them with some vaseline leaving a sweet spot roughly in the middle, but if I were you, I wouldnt worry about the holga inflexibility. You can still control film speed and by using a compensating developer, you can get reasonably reliable results.

I worte this piece for toycamera.com regarding this matter