Hello @TrippyLighting , please find my answers below:
1. There are two solid bodies in the design (see screenshot) For your purposes I assume that you only need the outer one. I would also assume that the rectangular cutout is also not needed. Can you confirm both questions?
Answer: You are right. I only need the outer layer. The rectangular cut-out is not needed.
2. In the file you attached I would assume that represent the original, unscaled form. The thickness between the 2 selected surfaces is 18 mm. If you scale that object down by factor 48 you'll end up with a distance of 0.375mm. For reasons that should be obvious , a simple thickening of the object is not going to accomplish much. How thick do you want to make this ?
Answer: You are right; the file I attached is unscaled. The 3D printer needs at least 2mm thickness across the wall. I prefer to have a 5 mm thickness across the panel to ensure that the part is strong enough (which is 240mm in the unscaled model). Thank you once again for your time and support.