While in this thing all Parts are available as separate files, in the customized things there's only one stl with all files inside.
I just spent some time trying different programs in order to find one capable of separating the parts. For everyone else who has this Problem: the Windows 10 3D Builder can do this quite equally. just ungroup, delete all but one, and safe. strg+z, and delete all but another.
I have no Idea if you can access the appropriate settings in the customizer, but I think for most people this might make things easier.
PS: Thanks for the great Design, I will post a make when it's assembled!