A dumbbell or kettlebell can be a good option for single-sided Romanian deadlifts, but if you're looking to really load those hamstrings (and glutes), using a landmine offers more stability.
The bar stays in contact with the floor throughout the lift. More stability means more output, so it can be a good way to bro-up your single-leg work.
Two ways to do it:
Landmine Side-Facing RDL
Landmine Front-Facing RDL
These work well with a variety of rep ranges. Try both options to find the one you like more.
Single-leg RDLs are most commonly done with a contralateral load: planted foot with the load in the opposite hand. Contralateral work mostly relies on using an interconnected line of tissues known as the "posterior functional line." This line connects the glutes of one hip to the opposite side lats, essentially creating an X-shape that crosses the lower back.
Why should you care? Well, think about athletes and their sport. Developing stabilization between hip and shoulder is essential. But if you care more about getting bulging hamstrings, contralateral might not be the best option.
Ipsilateral means using the same-side arm and leg. It's more useful when you're trying to train the muscles of the lead leg, and that hip "snap" from the lead leg you'll see in some sports. An ipsilateral RDL tends to be more stable and it emphasizes the lead leg's hamstring more.