Years ago I made a ground glass screen for a Medium format camera by getting two pieces of glass the right thickness and the same size and actually rubbing them together with fine white quarz sand between the sheets. A little water was added and the whole process only took 30 mins. The biggest problem is getting it even at the edges. This med format Rollie look-a-like still has one of the screens I made and it is working fine (although not very often now).