Reduce STL Wall Thickness

I have an STL whose walls sit somewhere between 1 mm and 5 mm, and I need the entire model uniformly brought down to about 1 mm without touching the outside scale, overall footprint, or fine details. In other words, I’m after an inward offset of the geometry rather than a global resize... (Budget: £10 - £20 GBP, Jobs: 3D Animation, 3D CAD, 3D Design, 3D Modelling, 3D Printing, 3D Rendering, Blender, Solidworks)