congrats on your first shop job, it's always super neat to make some money from our tools. I'd mill out the slot with a carbide endmill, then take a small test cut with a dovetail cutter (if HSS). My dovetail cutter is a DIY using TCGT inserts, so I would be less concerned with that. As above, use 2 pins large enough to rest against the face of the dovetail, about the same diameter as the depth of the slot works well. Then simply measure between the two in the known holder and cut until you reach that width with the one you're cutting. I'd go by eye on the first side, so you end up with the same thickness "lip" on the dovetail as the known holder, then cut the other side.
If you have problems with a HSS dovetail cutter, you can always cut the sides of the dovetail with an endmill by tilting the holder to the correct angle.