So this is my model in the preview. As you can see, many holes cut into the shape using a Boolean modifier. (Actually very happy with how it turned out). But when I render or export it as a .STL file, those holes are filled in (see below).
No idea why this is happening, I must have gone wrong somewhere along the way. Here is my .blend file uploaded to Blend-Exchange. If anyone has any bright ideas, I'd love the help.