To address some of the above posts, in my experience (which is two decades worth FWIW) there doesn't need to be any diffusion plexi used. I have always placed the mylar (or whatever else was available including paper) directly above the glass of the negative carrier. My current enlarger is a Saunders 4550 and the mixing box for 35mm is a combined diffusion/single condenser (similar to a Leitz V35). The technique works fine without adding any diffusion. The glass of the negative carrier provides enough space that whatever you put on top of it will be out of focus.