Sorry, I couldn't find any patterns for the seat itself, just for lid and tank. Here's one link for free crochet patterns, tho: http://crochetncrafts.com/crochet/shell-toilet-seat-cover.html -- I'm not surprised that nobody makes patterns for the actual seat, though. Putting a cover on the seat is impractical (as other responders pointed out) for hygiene reasons, male especially. -- Lord knows their aim sucks when they're half-asleep! (No offense meant, guys, just a fact.) I'd suggest getting one of the vinyl-covered cushioned toilet seats. We have them, and they only get slightly cool in cold temperatures, but never actually cold like the hard toilet seats, and they warm up from your bottom really fast. You can buy them really cheap at k-mart, Walmart, etc., too. Then you can crochet a nice cover for the lid, plus tank, if you want to. :)
Erm, what if a man got to it a bit late, and wizzed all over the toilet seat... then when you go to use it, it's soaken wet and colder than without a seat warmer :3 lol, I'd just get a plush-like toilet cover. You can get one from Walmart for like $5, lol.