The Lutron Aurora smart bulb dimmer is the best solution I've found to cover up an existing switch. Covers it very nicely, is extremely easy to install, is pretty easily removable, and
Lutron Aurora Wallplate Bracket for Paddle/Decorator Switches, for use with Aurora Smart Bulb Dimmer, White (L-AWALL1-WH) L-AWALL1-WH - The Home Depot
Lutron's new switch solves a big smart bulb headache, installs in minutes - CNET
Lutron Aurora Smart Bulb Dimmer Switch for Paddle Switches, Works with Philips Hue Smart Bulbs White Z-1BRL-PKGD-WH - Best Buy