As already mentioned, the shape of the focus patch is more of a fuzzy blob than a clear cut rectangle. But focusing is still very easy with a clean finder.
You can check how clean your finder is by holding the camera with the finder eye piece pointed towards a bright light source, then looking through the small window on the front of the camera. When you get the angle right, you can see a circle of light through the small window. If the circle of light looks clear and contrasty, your patch is as good as it gets. If the circle of light looks a bit hazy, your finder needs cleaning. The good news is that S2 finders are simply constructed and easy to clean, and improve dramatically after cleaning.
If it was me, I'd try cleaning the three surfaces of the half mirror prism just behind the small window. You only need to remove the front cover (4 screws) and the small window (3 screws) to access these prism surfaces. Cleaning the diagonal rear surface usually yields the biggest improvement to the RF patch. It's a bit tricky, but definitely not impossible to access from the front. I use folded lens cleaning paper slightly dampened with lens cleaning fluid. Below is what an S2 half mirror prism looks like when removed from the camera. The diagonal rear surface here is the one with the lens cleaning paper running along it.