Get some HSS and grind up whatever shape you require. Simplest to try would probably be to use the straight edge from the blank with the usual cutter reliefs behind. Think cross section of an endmill tooth.
I have a slotting head for my Thiel, but it is too heavy to change out the vertical head and I don't have enough headroom for a hoist, so it is lying dormant unable to be used.
Dave
The Emerald Isle