Ok. Here is my solution. Is cheap and is free and I used what I already have. I manged to adapt the Mamiya c330 original focusing screen + the clear glass onto the Yashica. I used to have a C330 and some additional focusing screens. I sold the C330 and still have one of the original focusing screen left. The Yashica focusing screen requires a ground class to focus. The Mamiya TLR does not. The problem is that, with the mamiya system, with the clear glass and the focusing screen, is too thick. The mamiya focusing screen is too close to the mirror. If the distance is too far, I can add shims. To get around it, I installed the mamiya focusing screen without the clear glass. Then I put the clear glass "above" the waist level finder. I used some blue masking tape, the 3M good kind, to tape down the clear glass. This clear glass isn't really needed and it is only used for protection for the focusing screen. Now, the focusing screen is too far from the mirror. That's an easy fix because I can add few layers of blue masking tape to shim the focusing screen.
Before I attempted this improvement, I put the Yashica on a tripod and to have it to focus on an object. Then, I marked down the distance scale when the desired focus is achieve. I used this setting to fine tune how much layers of masking tape I need to get the proper distance. Focusing clarity has been improved greatly since I am not looking at a ground glass but a clear glass. The brightness has improved too but nothing earth shattering. Let's say right now the image in the viewfinder looks very pleasant.
Let's just say that I can read off tinny words off my computer screen now.....