Well I feel a bit silly now - for some reason I assumed the libra 2 had a more square aspect ratio compared to the kindle paperwhite/voyage etc (I think the extra width from the page turn buttons skewed my perception) but it seems most ereader screens are 4:3. Knowing this, there are plenty of kindle screensavers available online that work perfectly for the libra 2.
This one is my current favourite find so far!