For RDLs, the Smith machine is actually superior to the barbell because you can sit back further since the machine holds you in place. This allows you to get a much deeper stretch in the hamstrings, and a more lengthened muscle is going to receive more of that delicious time under tension.
Smith Machine Romanian Deadlifts
Do 2 sets of 10 reps. Do the negative portion of each rep with a 5 second cadence.
Now, I personally don't like counting cadence in reps because I feel it takes away from what you should be focusing on – execution. So I'm saying "5 seconds" mainly just as a reminder for you to perform the eccentric very slowly and with control.
The hamstrings are built to handle the deceleration of the eccentric contraction. So there's almost no better way to build them than using this "non-functional" Smith machine... in precisely the way the hamstrings function.