Hi, does anyone have any experience with these backs?

I was going through flickr photos and noticed that a lot of polaroids made with a holga has severe vignetting. Yes, this is all apart of the appeal of a holga, but the vignetting is pretty severe, even for a holga.

On Freestyle's website, their sample images show this vignetting as well. Is there any way to remedy this? I'd love to get a polaroid back for a holga, but the vignetting is too much for my tastes.