QGIS has many fantastic plugin’s, but one of these I use most in my day to day work when creating maps is the Mask tool. This allows me to either completely, or partially, mask the base map outside my area of interest, so the user can concentrate fully on the features you have mapped within their area of interest.
It’s so simple. Just select the polygons you want outside of which you want the mask and then enter the symbology parameters. I personally make the mask completely white to remove the base map but you could alter transparency levels to really bring the base map alive within the polygon.
Also, I sometimes like to make the base map slightly lighter so as features you have added are not overwritten by the colour’s in the map. I do this by creating a shapefile, and then filling it in white but increasing the transparency to 60 – 70.