As for the cork float idea, I have lost maybe a $600-700 batch of wine that tasted perfect to tainted corks. It did not matter that I tried to sanitize them--they were bad through and through. I ended up tossing all the remainder of the corks I have on hand, and that was a sin. For that reason alone, I wouldn't recommend natural corks. I would consider something like Supremecorq, however. If you want, I bet your LHBS has them in stock. Mine has them in a bin. Sterilization? Well, since the material is reportedly similar to those found in artificial hearts, it should be safe for beer.
I have used the mesh hop ball thing like so many others here have listed. 90% of the time it was fine. The only downside was once I held a party and it did end up clogging the damn tube. Its always when you're entertaining that you have the issues...
I like the magnet idea. Pretty clever you hombrew folks are, aren't you?
Best,
MMJ