I had the good fortune to be given a Chillcheater emergency hatch cover recently. They are simply the cat's meow of emergency covers. The fits is totally ruggedly tight and sealing. They are very UV resistant, have a built in tensioner making them adjust to different hatch sizes, and have a means to tether them to the boat. They have all sizes, round and small.
Just wanted to pass this on and to see what other products people have found to be good.