I've flown on plastic floats and built up floats and foam floats. Foam floats can't leak, they're light if you don't slop on a lot of paint, they are easy to repair and they last a long time.

We normally use water based polyurethane varnish and very light fiberglass cloth, but have done them with Monokote. I have seen them covered with colored tape.

I liked the Prentice foam floats sold by a man up near Niagara Falls, Canada, but he has closed down. I am trying Plane Fun Floats for a new plane.

If you want flat bottom foam floats, here is a place that sells them. You do have to sheet these. [link=http://users.cwnet.com/zdgreg/]John Sullivan floats[/link]