Add a Fresnel to 617 groundglass?

I have a Fotoman 617 camera with Nikkor 90/8 lens. Viewfinder is not accurate, so I compose on groundglass. Because of the light falloff I can not see the whole image at the same time. Yes, I can see corners if I move head left/right, but it's not the way I could compose precisely. I really need to see the whole image to evaluate the composition.

Is there any solution for this? Maybe an add-on fresnel or another GG (Maxwell)?

Ambroz

To try a cheap solution you could buy and cut the cheap plastic Fresnel lens from a paper shop and see for yourself.