My observation is that built-up floats are heavy.
Blow-molded plastic floats (GeeBee) are kinda klunky, but people either seem to love them or hate them.
Fiberglass floats are pretty and expensive, and get fractured, and frequently leak at the seams.
Foam floats can't fill up with water and sink, and are durable. I have one set on their fifth airplane. SHeeted with balsa, except for the points on the bottom that take the shock loads: inserted 1/16 ply sheeting just in front of the step. They're not unscratched, but they are still serviceable. I don't know of anyone making foam floats commercially, now. BJ was the preeminent brand. I generally charge $35 plus shipping for a set of v-bottom, tapered-side 45" foam cores that look a LOT like BJ floats....kind of a standoff scale EDO float. The nose is not as tapered as an EDO, but that keeps the prop out of the water.

Here's an article about making simple flat-bottom foam floats. You can get 4' x 8' x 2" sheets of foam at Home Depot for $10. Laminate it until it's as thick as you need.

http://www.flyinglindy.homestead.com/skisandfloats.html

There are several other articles on my Florida Float Flyers website: (That's why it's there)
http://jcasey02.home.gate.net/fff.html