My favorite diffusion material is 1/16 inch "Sign White" Acrylite (similar to Plexiglas). This transmits more light than standard translucent white acrylic of the same thickness. If falloff correction is needed, this material can be ground thinner towards the corners, but that's a tricky acquired skill. So just burn in the corners and edges when using a 150, or use a longer enlarging lens like a 180 if you don't want to burn-in. No big deal.
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