There are tray-type racks available for receiver hitches that hold the bikes by the wheels, leaving the frame untouched and undamaged and the bike sitting level. There are also frame-arm type racks that have arms with adjustable heights to accommodate sloping top tubes.
I would imagine you could bend one or both arms of a fixed arm type rack that has both arms at the same height or block up the head tube end of the top tube with a foam spacer.