Many/most RhinoMoto bar end sliders won't work (too thick). They have one S2R/S4R/848 specific model for your tapered aluminum bar, which has inner diameter of only about 14mm. It's very possible the vendor sent you a wrong one, if they aren't used to Ducati or not awake at the time.
So, I think, if you specified your bike and they still sent you the wrong ones - they should pay the coming double return/shipping cost.
You're not stupid, you're just learning to double check everything you do
![Wink](https://ducatimonsterforum.org/Smileys/classic/wink.gif)
Which mirrors did you get?