I have an automatic door that slides up now, that works great. It's a metal door. If you like the sliding style of door and the warping is the only problem, you could use a metal door.
Before we put in the auto door, that coop had a hinged plywood door. I designed the coop for deep litter, so the door is elevated, to keep litter inside. I hinged the door at the bottom and the door opened down into the litter to make a ramp. You could also make one with the hinges on top. I frame in openings with 2x4s. This gives it stability and also a sturdy place to attach hardware. I like wood screws for attaching hardware, so they can't be pulled out, which has happened to a few people.
To lock my hinged door, which was on the inside of the coop, I just added slide latches to the two sides. In my case, a latch at the top would have worked, too. If you hinge yours at the top, you'll also need a way to lock it open. If it's installed on the outside, you need a latch or lock that raccoons can't open, to lock it closed.
To prevent warping, I use exterior grade plywood, paint with exterior grade paint and have large roof overhangs to keep rain off. I don't know if you did all of that and still had problems, but that worked for us. I know a sliding door is more touchy than a hinged door, especially if the track doesn't have much extra room in it. We also have a lot of ventilation to keep humidity down in the coop, although the summers are still pretty steamy.
