Which rack are you using now? I can give you some personal recommendations from the multiple ones I've used and owned.
Older Yakima that has three straps. I have to really wrench it down to hold the bikes. Not wanting to put the new SC on there.
So I highly recommend a hitch rack if you are not using one now. They are the most secure racks and you get plenty more options. My current rack is a Kuat Sherpa 2.0, works really well. But I'm moving to a new Saris MTR soon and I'd highly recommend one after checking it out in person. Basically any tray rack works the best where it doesnt contact the frame for the best hold. Thule T2, Saris Superclamp EX have also been used by me in the past and work well.
Mitchell Mathews | Community Manager | Trek Bikes | @mitchmathewz