I am trying to edit an STL file for 3D printing through Solidworks as the software we use at work (Meshmixer) is hopeless.

I want to create simple hole in the bottom of the model using extrude cut, but the features are greyed out as the STL is just a heap of surfaces. So I have tried to import the STL as a solid (which takes about half an hour), but keep getting error messages saying it can't make it solid. Surely there is some way to make a surface body solid in Solidworks, I just don't know how to do it.

There is also and old part file of mine that Solidworks also includes in the model when the STL opens for some reason.

Can someone please explain how I can convert my file into a solid so that I can cut extrude it in Solidworks?