Thickening a complex .stl file for 3D printing

So I have purchased some .stl models which I am scaling down from 1:1 to 1:6 scale (16% of original) to 3D print on a resin printer.

Typical problem, wall thickness is way too thin after scaling.

I have seen videos about the “Thicken” command in blender, but they only use simple boxes or cylinders. I want to keep the outer detail untouched, but only want to thicken the inner part of say this leg armour or helmet.

No one will see the inner side, so it doesn’t need to be pretty. Just thicker.

Would love some help here. I’m sure I’m not the first to have encountered this problem.

Is there a way to localise internal thickening rather than selecting the whole model ?