You could try one of those plastic clips, which give you semi-crossed bra straps.
I sometimes do this when I am wearing a sleeveless top. Only a couple of my bras have straps which are long enough, and it's a real struggle to get the clip engaged across the middle of my back without help.
It's also quite uncomfortable, as the straps cut into my shoulders, but you are really harnessed-in and nothing can cause you to drop or flop!