I had EZ Brackets on the original R3 bags and this worked fine, except that the backing on those bags wasn't rigid enough.
With the large cutout, the Slimbag is a pain but you could mount a rectangular aluminum or plexiglass spacer between the bracket and bag and attach the bracket to the spacer if you can't get a purchase on the bag with the bottom of the bracket.
With the large cutout, the Slimbag is a pain but you could mount a rectangular aluminum or plexiglass spacer between the bracket and bag and attach the bracket to the spacer if you can't get a purchase on the bag with the bottom of the bracket.