Rick,
You are right regarding a hard top traler, need about a 20 for a 15 foot coupe.
But a soft top, a 15 foot coupe can be towed in an enclosed 16 foot Serpent Express. Same way a 13 foot roadster is towed in a 14 foot SE. The front and driver's side opens for tie down access.
In this image you can see the front open and the sides. The right side door is optional.
With E-Track tie down system, doors are at the front wheels.
Hope this clarifies.