This photo shows the prop sitting cleanly on the Saber extension I have. Two of the bolts were a clean, drop in fit, two would go with a "little" persuasion and the last two not at all.
Solution ... refill the holes and re-drill them. I came up with a highly precise alignment tool. The 6" Saber extension is CNC machined so it couldn't be off at all. I filled the holes part way up leaving the dowel holes as these ends were already correct.
I had a friend run a 1/4" drill through the centre of two AN6 bolts on a lathe and then inserted these in either end of the extension. Now when I passed this extra-long 1/4" drill through these guides it was held at two points about 6" apart and precisely aligned. When finished 5 bolts dropped straight home and the last went home with a slight push. Happy with this result.