Stl to Solid

Refer to this article for some options.

Article - CS69498 - How to create solid model from an STL or WRL file in Creo Parametric

Any solid model created from an STL file is going to be a faceted approximation of the "real" part. If you're just using it to check clearances, or to maybe make a 3D plastic prototype, that's probably okay. If you want an actual model that has "nice" surfaces you have other options like

(a) Read in the STL file and use its geometry as a guide to build a proper solid model. Not fun, but it'll probably suffice, depending on how precise the surfaces of the model need to be.

(b) Contact Dentaurum or whoever and see if they will provide you a model. Not likely, this is something they sell and they probably already have trouble with counterfeiters (everyone does).

(c) Buy one and measure it to build your own model - I've done this when I needed to modify a "stock" part for my needs.