Back to the OP, the cheapest and easiest is a 5 gallon bucket. Someone mentioned foam pipe wrap around the rim. Great idea. They also sell plastic toilet seats that are made to fit over a 5 gallon bucket. I keep one in my cabin for the winter months. Wife and daughter aren't a big fan of walking outside when it's -30 or -40 degrees. I keep a garbage bag in there and they sit on the seat. My 8yr old daughter loves it. When not in use, the bucket goes outside while the seat stays warm inside. For a boat, just do the same and throw the bag into a dumpster at the end of the day.