I'm a fan of using painters tape.... pretty low profile, easily removed, stays on really well. Also the weird blue glow you get when the camera is on tells you pretty quick "Hey there's a cover on there" where sometimes with the all blackout covers ... I can't tell.
I renew my calls for all devices to have an led indicator (good on Apple here) and a physical switch that cuts power to mics and cameras for all devices with them. With the endless layers of software we have today, I have trouble trusting anything but cutting power.