Hide-a-mic B Flex Body Pads

These grip the silicone of the Hide-a-mic and stick nicely to skin for hours. They're cut to the right size and can probably be reused a few times. (It's more expensive than double-sided tape but also the right shape, grippiness, and skin-friendliness.)

Add your own strain support (loop the wire and stick medical tape between the loop and rest of the cable) and you have totally hidden, high quality sound for a full day of recording even under a thin t-shirt.