Unfortunately, here, I think the problem is that English is not very clear.
As a native English speaker, the first time I encountered the word 'draft' was at primary school. We used it to describe a piece of writing where presentation was not the focus, instead, content and accuracy in terms of spelling, grammar, and punctuation would be the focus. Once the drafts were complete, we would 'copy these up' in our neatest handwriting.
However, if I prepare a 'draft' of some document or other for my boss, I expect it to be essentially an unapproved version of a final document, perhaps needing some minor modifications before release, but also perhaps not.
Although personally, I would use 'sketch' to mean something disposable which illustrates a more perfect version, my grandmother is an artist, and she refers to the initial drawings she makes on the canvas as 'sketches', which she then paints over in more detail.
Essentially, I don't think there's a big difference between draft and sketch - both could (in my opinion) represent either a version which will be discarded or which will be developed further.
Overall, I think here, the 'Work In Progress' label is the clearest and least likely to be interpreted differently by different users.