After not seeing a usable set of French doors in months I have found 2 sets in week. The first set are aluminum framed which is probably practical for the exposed side of the cabin where they would fit, and they are slightly narrower so they will fit easily in the gap. The gap in this case is exactly 1666mm.

The second set of doors are wood and will need sanding, painting, and regular maintenance. They are also 1640mm wide without the jambs, so in total I will need 1676mm to install them. I’m leaning towards the wood pair just because I am more comfortable cutting up wood and installing hardware than I am aluminum. I suspect the greenhouse is going to get a nice set of aluminum doors and I am going to be planing 5mm of both edges of each of the wood doors.