Frosted translucency? I think you will need render-to-texture to make that work right.
Create a texture render surface where the door will be. Create a render-to-texture camera where the regular camera is.
Then use this as an input into a material that makes a milky translucent surface.
Apply lots of blur.
Maybe there’s some additional magic that lets you do this single-pass; I’d be super interested in learning about it if there was!