Like everything else, what are you going to do with it and how long do you expect it to last ? Water tight or just to keep debris out ? Just keeping debris out and most of the water, then an over the counter general purpose will work. You will need to add extra support to keep the cover tight and will not last long if trailered. To get a good, trailerable cover that sheds water and will last, requires a custom made to fit for your boat cover. Yes they are expensive but they work and they last. The last custom cover I had made was made from convert able car top material, looked like new after 10 years when I sold the boat. Never leaked a drop, trailered all over, but kept clean and maintained. Expensive yes, but well worth the cost as I got my money back when I sold the boat. You will get what you pay for !!