I bought these last week:
I tried really hard to get rounded molds that were oval shaped but not flat. These were the only ones I found, and came with a lot of extras I didn’t need. They also turned out to be extremely flimsy, so I’m a bit skeptical.
Otherwise, I’m really happy with these half sphere molds also used for cakes:
They come in different sizes, I have 7 cm; found it in a store for cooking supplies.
Semi spheres are really cool, because they have a property of “unstable equilibrium” which means that you can balance them on their round tip, and they might wobble but they won’t flip over. So you have the least amount of surface area possible touching the soap holder, and the water can just dry off. They’re also nice to handle; as a body soap you can cradle the curved part in your hand and use the flat part to clean. Highly recommend