Cheap way: get a hole plug with the finish you want. Drill or punch a hole in it. Does the faucet really need a 1-1/4 inch diameter hole? I would have thought a smaller hole would work.
You could look around the sink drain fittings. Maybe you could find a slip joint nut that would be used.
Fancier way: buy a new RO socket that is made for 1-1/2 inch hole. Some are not so expensive. Some have built-in TDS sensors.