Best answers I've heard or devised so far:

1. Leonhart's suggestion below. Probably the best rhetorical move Harry could possibly make.

2. Harry's portkey is transfigured into a tiny chip implanted under his own skin. It would be totally in character for him to do that.

3. If he has the range, transfigure long, thin tendrils that overlap with Voldemort and the death eaters' spinal cords at the neck level. Thin so that he doesn't have to work with as much mass/volume.

4. Transfigure the air around him into a carbon nanotube shell, buying himself time for another spell.