Perhaps you could sew in a piece of sheer material or a lacy one which you can buy and hook across bra straps. Other than that get a light necklace, a long one that you can wind round several times.
I have some pieces of material called modesty panels. The have a lacey edge and they clip to bra straps at each side. I have one black, one white and one pink. I put them on when my top shows too much flesh.
They have plenty on ebay and are only a pound or so each.