How can I cover a hideous birthmark and make the make up stay?

there is this nasty, and I mean really gross - I've had it checked out by the dermatologist and she said that its not cancerous but its hideous, birthmark in the center of my chest and its about this >>> O <<< big and I can't figure out a way to cover it up in order to get in a bikini for my mexico trip in two weeks! helpp!!?