Fit a dioptric correction lens to your viewfinder eyepiece. Apart from using a viewfinder magnifier and/or progressively larger corrections in order to see the "sweet spot" clearly, dioptric corrections are the cheapest and most effective way to compensate for eyes if you don't like wearing glasses (that's me!). Most viewfinders can also be accurately focused using the matte surround, something I often do with the Pentax 67 in poor light. Depth of field will take care of slight imperfections in focus.