Best Of

I think your end result is to have a box with walls and bottom of 1.2mm thickness. If so, just extrude the entire sketch up 1.2mm and then use Extrude/Thin on one of the perimeter sketch shape entities. Which way (arrow selection) to direct the extrude depends on which perimeter you choose. - Scotty

Thanks! I'll look at that option now. Did I go about this correctly? I spent a lot of time adding the inner boundary, but maybe I could have kept it a solid shape with a single line, and somehow converted it to a hollow object with constant wall thickness?

I just updated sharing to allow onshape users to copy etc.

The link you provided is view only so it's not possible to examine the feature tree to see what you have done to produce the results you have. Just glancing at the feature tree, it looks like there is an abundance of extrudes. If possible, please make your document Public so we can take a peek. - Scotty

One more question if you don't mind. I have completed the box but I don't know how to remove the top accurately. If I try to extrude/remove into the box it takes the walls off as well.

This sort of part would probably be much easier to make with a shell feature at the end.

Not sure what your calling the top, but just select whatever face and extrude cut it through the box.