Hey Pat!
Curt only makes the 21" Extension # C18117 for their Roof Mounted Cargo Basket # C18115, so I recommend using the standard size for your 2015 Buick Encore.
If you don't already have the cargo basket then you might be interested in the Rhino-Rack Roof Mounted Steel Cargo Basket # RMCB which is a little bit longer and has a lot more adjustability for the mounting brackets. view full answer...