Have you tried the RuffWear harness? Their stuff is great- I don't think she'd get out of that. Or you could get one of their backpacks, which all have the same underlying harness and she can carry her water and stuff too!

The other ones that might be less slip-able are the Freedom/Orion style harnesses- tend to be more stretchy and wider than webbing, so would take more to get out of I'd think.