Not perfectly cut, but I finally replaced the stock silicone sidegrip! (Steelseries Rival 600 w/ Paracord, Hyperglides and GT-5000 Griptape) : r/MouseReview
Amazon.com: Mouse Side Panels/Anti-Paste/Side Grip/Anti Sweat Paste Grip Tape Replacement for Steelseries Rival 600 / Rival 650 Mouse : Video Games
Amazon.com: Mars fox Mouse Side Panels/Anti-Paste/Side Grip for Steelseries Rival 600 / Rival 650 : Video Games
How to Re-Glue Rival 600 Rubber Grips (With Pictures) : r/steelseries