"I once slightly reduced the horizontal size inserting a small inductor in series with the yoke. "

That method affect the linearity. Sometimes it is acceptable and other times it is not.The only way is with a bucking winding. Sometimes you can find a trace positive winding on the flyback feeding a scan derived source but this is a direct line set, no power transformer. Hot chassis. That means it would cross cold and hot grounds and that is a big nono. Finding the right coil might be a problem as well, but with a tertiary winding on the flyback you can add or subtract turns to get it right where you want it.

Note that this also lengthens retrace time but not as much as adding capacitance across the primary.