I downloaded this grip, and modified it to slide onto the joy cons much easier, with the right settings, you shouldn't have to sand anything down, which was the main complaint on Print3DLLC's print. I printed it straight up with a raft for stability. Also, I took a box cutter and cut out a slot so that the joycons lock into place. (added a picture)

I mostly use this grip for Pokemon Let's go! With this grip, i can use both joycons with a single hand, since it is only as wide as a cell phone.