How about stitching the bodysuit down to a padded bra? Could also run a piece of elastic from your neck down to the bra to stitch to as well. How thin is the fabric you will be using that it will show a bra underneath? O.o I used the stitch down method for the white boob plate in the following costume but I have yet to try it in a full bodysuit.
Just call me Rhetti. Really. Psycho is sooo high school.