Just got back from a week in Port Douglas. We did Wavelength and it was great. I did book about 1 month in advance, it does fill up as they only take 30 at a time. If the weather is bad they will rebook for another day. I booked our trip for the second day in PD just in case weather was bad we had 5 more days to replan. We did not have to replan, weather was great and we had a great day. I wrote a review about them.
Not sure if you are driving to Kuranda or to the skyrail station. We also had a car and drove to the skyrail station, about 45 minutes from PD. We actually booked from a place in PD that had the skyrail and train package for $93, we had to put 10% deposit down and paid the rest when we got to the skyrail. We only did this the day before. So no need to prebook, you can also just buy your tickets the day you go to skyrail. Once you get there it is easy to figure out what line to be in and it moves fairly quickly. Then you are on your way. If you take the skyrail/train combo you will end up in a different place from where you boarded skyrail. They have busses, with aircon, that will take you on about a 5 minute ride to the carpark at skyrail. It is a realy easy process from start to finish. We got to the skyrail at 9:30 and the train from Kuranda left a 2, this was plenty of time and we were ready to head back. They skyrail makes 2 stops, your choice if you want to get off and how long you want to stay. We got off both times to see the views, I would recommend this. I also wrote a review about that. Okay, didn't mean to write a mini novel, hope I answered some of your question. Have fun in PD.