Thanks for all the answers!
I am aware that adding space between the GG and the lens will add a focusing error to the camera, being a fresnel or other thing!
I had a toyo with a fresnel and i removed it since the image did not seemed a lot brighter and a hot spot was added, specially with the 90mm. Although the ground glass and fresnel were from a horseman camera... but that doesn´t seem important to me
the mais question (and the cheapest option...) yanke or hopf....