Need affordable modifier for Rembrandt or butterfly lighting with a speedlight

[Patrick90]

I'm an amateur photographer trying to improve my indoor portrait work, and I feel like my lighting is holding me back. I have a basic speedlight and a reflector, but my results are often flat or harsh. I want to learn classic portrait lighting techniques like Rembrandt or butterfly lighting, but I'm struggling with the practical setup and modifiers. For those who shoot portraits regularly, what is the most versatile and affordable lighting modifier you'd recommend starting with to achieve more professional, flattering light?