Xgrip, with just a little bit of sanding in the corners to remove the little bit of resistance.
Also, a discovery I just made last night. lots of people have been using a P2000SK baseplate with the finger rest with the same light sanding to make it fit. I had a USP-C/P2000 9mm one laying around too, and that one fit with even less sanding required than the P2000SK ones. They're a slightly different mold and it seems to fit better. Technically it didn't even need sanding, but was just a hair tight.