Thicken STL Models For FDM

I have 6 small STL files—each under 3 MB—that need a quick update so they print flawlessly on an FDM machine. The job is simple: reinforce one entire side of every model by adding material while leaving the rest of the geometry and key dimensions untouched... (Budget: €8 - €30 EUR, Jobs: 3D Animation, 3D CAD, 3D Design, 3D File Scaling, Slicing & Print Preparation, 3D Modelling, 3D Print Design, 3D Rendering, 3ds Max)