Hi Christel, here's a Hometalk post that may help you! https://www.hometalk.com/39224279/diy-rollable-cedar-hot-tub-spa-cover
If you live somewhere that gets really cold, a homemade hot tub cover seems like a bad idea. It keeps the water from freezing in the winter, as well as helps maintain the temp. Look for a used one on craigslist or facebook marketplace instead. A good one shouldn't weigh much. If it's super heavy, that means it's saturated with water.
There made of styraphome in a cover
i think it would be like any other surface. you would need heavy duty vinyl. make an opening to pull through all the way around and then run a rope...you would use a very large heavy duty pin to do this. make it large than you need and then pull rope tight to cover. this may be a bit expensive for the vinyl or water proof fabric., but worth it.