Hi Lorenzo. I always use an external viewfinder. I have several of them:
- Helios viewfinder with brighlines for 35, 85 and 135mm. Those are the smallest and cheapest option I have found. They feel quite sturdy too. Nowadays they sell around 30-40$ on ebay, but I got mine for less when they were still available from distributor. This is the viewfinder I'm using all the time with my J-12 lens.
- turret viewfinders. This is a viewfinder with build in lenses for different focal distances. It is rather heavy and bulky, its quality is great though. Those usually cost around 70$
- another type of viewfinder that I have is Walz viewfinder with adjustable frame border (you can rotate the ring and the black border is expanding or contracting to change field of view. It also has parallax correction. I like this one a lot! But those are really rare. There is a similar Leica viewfinder as far as I remember, but it costs a lot. It has similar size as the turret viewfinder. The advanatage is that there are intermediate field of view sizes, not only the common 35,85,135mm ones.
I haven't tried FSU dedicated 35mm viewfinders, people say they are good.