Apple - Fill rectangle with diagonal lines (stripes) in keynote

After some searching about, it would appear that you cannot do this in Keynote; however, you can import an image (like the one in your question) and then do what you will with it.

Old post, I know. Just searched for the same question. Read this post, did the following.

1. Open blank keynote document
2. Draw a vertical full height line, 2 point, in my preferred colour.
3. Duplicated the line a dozen times
4. Selected all and duplicated the selection a few more times
5. Used align to make sure all lines were vertically positioned the same way
6. Moved one line to the right edge of the slide
7. Used distribute to set the distance between objects
8. Used rotate in the inspector to rotate all lines 45 degrees
9. Exported as a pdf
10. Opened my original document and dragged the pdf into it
11. Double clicked and used default keynote tools to mask to the shape I wanted

Hope that helps someone else.