My brother has a convertible and he just rides with his dog in the passenger seat (he has a lab mix, around 60 pounds I'd guess)
His dog wears a harness and he ties it to the bottom part of the chair. Sort of like a ghetto dog seatbelt lol
With Romeo, he just sits in the passenger seat as well, usually on somebodys lap. It's A LOT of fun and most dogs love it (it's like sticking your head out of a window lol)
My bros dog also has a pair of doggles, since he rides in the car with the top down all the time and he didn't want to risk him getting something in his eye or something.
To those who don't know what doggles are lol
It's a regular thing, they go to work together everyday, or the beach on the weekends.
It's not a pain at all, at first it was hard getting him to learn to just SIT and RELAX in the passenger seat (especially when he was a puppy) but they did slow test drives around the neighborhood at first and now he just sits there and ignores other dogs on the sidewalks and stuff.
And the car is a two seater.