Thank you for your input Dave!
I have thought about this option too Tjark. The only issue is that for small keyways, for example 2mm the specified hole is between 6-8mm. This means that my adapter that holds the cutter should not be bigger then 5.5mm in diameter. It will be impossible to hold a blade like this firmly. If I would grind such a blade to size I am afraid that it will break in the cutting action.....
Up to now the most rigid option I can think of is to make a round adapter, drill a perpendicular hole, grind a piece of round HSS bit to shape and clamp it in the adapter by a locking screw.... hope my idea is clear....
Regards Jeroen