The keys to great putting are a proper face angle at impact and an optimal path for the putter, at least according to Jon and Jim McLean, developers of the Slot Trainer.
The device aids each of these factors by creating two “gates”— one for the ball to roll through, the other to align the putter at address and impact.
The Slot Trainer (only 1/16-inch thick so as to not interfere with the stroke) is designed for use almost anywhere: practice greens, bedroom, office, you name it.
Several Tour pros have used it, including Brooks Koepka, Charley Hoffman and Greg Chalmers, who happened to finish third in strokes gained putting on the PGA Tour in 2014. $50; eyelinegolf.com