In addition to the Rosco kit you mentioned it might also be worth taking a look at LEE filters. They offer a visual comparison of their diffusion materials.

There is also a quite useful comparison of different bounce and diffusion materials by Matt Porwoll.

While these demonstrations are mainly intended for larger-scale photographic and cinematographic use, they should give you an idea of how they will perform in your situation. For starters a filter set with some ranges of white diffusion and frost will get you started and it's useful to have them in general.

There's also white diffuse acrylic glass, which would probably be a more durable solution.

Be aware though that the relative size of your light source will influence the characteristics of your lighting, so you should make sure that the 20-30cm diameter is sufficient for your subject (as a rule of thumb, to achieve „soft“ lighting your light source should be at least the size of the area you intend to light, and that's assuming you are up pretty close to the subject. Usually you'd want to move away from the subject to reduce light falloff, which means you'd need to scale up your light source as well to achieve the same shadow characteristics).

As a side node, in still photography you might get higher light output from a flash.