What are my options to fix a negative canthal tilt? (Photos)

The only effective way to permanently and in natural way make eyes more positive canthal tilt is to raise the lower eyelids in addition to the canthus; that is called lower eyelid retraction surgery with canthoplasty (almond eye surgery). It is not just canthoplasty alone. 

There are surgical techniques to lift the lateral canthus, or the attachment of the lower lid to the outside wall of the orbit, but I would recommend adding volume below this area rather than a surgical procedure to give the appearance of a well rested, alert eye and brow rather than immediately proceeding with surgery.  This volume may include hyaluronic acid filler, fat injection, among others.

From your photos you can help that by elevating your lower lid with a canthopexy or canthoplasty.That should help a lot.