Excellent covers - Bravo Company Mfg KeyMod Rail Cover Kit, 5.5-inch 5-Pack

Honestly, just really great. Slim, low profile, rigid, and very grippy texture while also being durable and affordable. Everything else out there is either flimsy, too rubbery, expensive, or bulky and weighty once you have your whole handguard fitted with them. These are the perfect mix of everything and are even vented if you have something that gets too hot and needs to breathe. It's worth buying two packs to use one set for cutting and sanding down to fit between other accessories (just try to keep them at a minimum of two slots, one-slot pieces are sorta wiggly).