RE: great planes/goldberg floats
I have used nothing but foam-core floats for YEARS.
First set were BJ-no longer available
I learned to cut my own. Made quite a few sets for my friends.
Now I get my floats from PlaneFun Floats or Seaplane Supply
Foam floats are much lighter than the ply floats, durable, and they can't leak enough to change your CG. They ARE repairable.
I once overshot a landing and tore up the bottom of some floats on some riprap. A little packing tape, and I flew the rest of the afternoon.
If you have a set of ply floats that leak, in a week or 2 you will have a science project growing inside. (ick!)