Good film* cameras had interchangable viewfinder/focus-aids that are just an etched piece of glass in the viewfinder optical path. You'd ideally want to etch cr

Good film* cameras had interchangable viewfinder/focus-aids that are just an etched piece of glass in the viewfinder optical path. You'd ideally want to etch crop lines onto one of those. * And digital SLRs, most of the higher end Canon and Nikon dSLRs can take a replacement viewfinder glass that adds a manual focusing ring or suchlike, they're usually cheap about £30 last I checked (pre-covid), but fiddly to install.