There's always a trade-off between privacy and convenience. I recommend metal slider covers that are easy to open/close as needed. CloudValley makes good ones - they're better quality than other brands I've tried. They stick well if you're reasonably mindful with phone handling, and they're inexpensive to replace as needed.

can also be done with the mics, but you have to actually cover the case and usually there is one top and bottom of the phone... doesn't work to cover up the secondary holes if it's got a vinyl bumper sleeve thing like some phones have... sealing them does the trick, they become deaf to everything except bumping the case