After quite a few disappointments, we finally found a harness that works well. It is the Kinklab low rise. We bought the full curves size, as my SO is a bit curvy. We've had several harnesses before that just weren't secure enough, but this one is solid and allows spirited play. It allows for a lot of adjustment, and she is able to position the dildo exactly where she wants it.
It is pricey, but if you look on Amazon, you can find it for under $50.
We paired it with a Mistress dildo that was suggested by another poster here (thanks oldkinky). It is just about perfect. I found it at a decent price on shevibe.com.