As long as you've got a roof rack system on your 2017 Cadillac XTS, you can use the SportRack Horizon # SR7016, which fits on virtually any crossbar shape. It doesn't look like the XTS came with anything other than a naked roof (no side rails or factory crossbars), so you may need the "extra supports" you mentioned if those are not present.
If you don't have a roof rack system on your vehicle, you will need to add one. For that I recommend using the Rhino-Rack ROC25 Roof Rack for Naked... view full answer...
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