Modify STL Inner Dimensions

I have an existing STL for a small, DIY-3D-printed peristaltic pump housing that I will be printing in PETG. The outside of the part already fits perfectly with the rest of my build, so the outer wall geometry must stay exactly as it is... (Budget: $10 - $30 CAD, Jobs: 3D CAD, 3D Design, 3D File Scaling, Slicing & Print Preparation, 3D Modelling, 3D Printing, 3D Printing & Rapid Prototyping, 3D Rendering, Blender, CAD/CAM, Solidworks)