top tube adapter for hitch mount carrier
Yakima and Thule and probably most other rack companies have top tube adapters. They all recognize the need to accommodate the sloping top tube/y frame/children's bike market. They will all function relatively the same. Unless you have an ultra lightweight delicate carbon frame/seatpost, which I don't believe your bicycle qualifies for that, you should be fine.

Most of them are adjustable so you can make them fit snugly and securely.