Just to be sure:
You definitely want to stay at the Westin Cancun Resort and not the Westin Lagunamar, right? For the latter you might be able to find some 3rd party time share rental deals.
(Wouldn't give you any SPG perks though, obviously).
Otherwise check the deals as mentioned by travelswithmyself. It might be even worth a shot calling SPG to find offers which are not published online.