Simple modification to Thingiverse STL file

I have a Thingiverse mounting case for an Invoxia GPS tracker (PNG attached below). I need the mounting face of the case to be thickened to accommodate two recessed mounting screws such that the screw heads won't obstruct the central cavity of the mounting case. Recessed mounting screw holes need to be added to the the mounting face, and two aligned access holes on the non-mounting face of the case need to be added to allow allen wrench access to the mounting screws. I can sketch these modifications on screenshots of the STL file along with dimensions for the hole locations, recess depth, and diameter.