Good post, stales!
I'd like to add that you can buy a yard of naugahide (sp?) or vinyl at a fabric and/or craft store probably pretty cheap to cover your seat. You might also want to buy a piece of foam while you're at the store to add some padding. Cut the foam to fit the top of the seat then glue it on with some Aleene's Tacky glue (they sell it at most craft stores). If you tell a salesperson at the craft store what you're trying to do, chances are they'll give you some tips. Maybe you could even bring the seat in with you and they'll tell you how much material you'll need.
One more thing--whatever material you use, make sure it is fairly strong and waterproof so it will last.