Top tube adapter. They're widely available, inexpensive, and then you're not clamping the rack straight to your top tube.
I understand-- yes, tray racks are better. But I have a 3-bike Allen rack, and it fits perfectly in the trunk of the wife's car when not in use, has never had a single issue transporting our bikes, and didn't cost $400.
So if the wife needs to pick me up due to a mechanical or any other issue, she just drives to me, the rack is in the trunk with it's bag of tools, ready to go. With a tray rack sitting in the garage? I'd sooner walk home than ask her to try to mount it.
Oh, and a thumbs up for the Allen racks-- the strappy bits aren't the best (they're the cinch-type like backpack straps rather than stretchy rubber) but they're plenty secure.