I did this about 3 years ago. There wasn't any door on the carriage house but it was framed in for a door. I built mine from 3/8 plywood siding with a 1 by 4 lumber frame around the perimeter on both sides screwed together for rigidity. Also some extra 1 by 4 on the front in a decorative pattern that helps add more rigidity. Each of the two doors are almost 8 feet wide and pretty heavy. I used big hinges available from the Borg. I guess they were gate hinges. They mount on the surface and there are 3 of them on each door. If interested I could post a picture. Because of the weight it is a must that the hinges (whatever kind you use) are screwed into framing members and not just though the trim. Also, I found out after the fact that I should have allowed more clearance on the bottom as the cement pad outside wasn't exactly flat and the door rubbed and bound up on a high spot. I'm planning on cutting a bit more off the bottom and finding some kind of rubber strip that will seal out the leaves and such. I used an idea I saw online somewhere to use a regular garage door opener to open and close the door. It works very well. They have been up about 3 years now with no problem except a sag on one side because I didn't get the hinges on that side into framing.