It might be helpful to put some numbers to this. The relative proportions are there, but overall size of the parts is lacking.
A hint at the volume might matter too. If you are only doing one, it may not be worth building a jig, but for more than a few, it can really help.
For a single (or opposing pair) it can be as easy as clamping a third piece alongside of the thin one, clamping it in place, and referencing the base of the Domino against that "extra" piece. This will put the slot 10mm above that spot, which is why I asked about size. Then just place that thinner part on the benchtop, with the mating surface facing the Domino, and cut the other slot.
At this point, you should be able to assemble the pieces, with a tenon in place, and the piece will fit right next to the one that is still clamped to the bigger one.