Can someone tell me what is the best material of foam to build a rigger sponson?

closed cell may be best.

One of our hydro guys and the designer of the KEPS T2 swears blind by Divinycell H60 or as an alternative Airex C70.55. Other alternatives, XPS foam, Dow (blue) or Corning (pink).

I use a 4.5 or 6 lbs density Polyether Polyurethane foam with fine closed cells for sponsons. The 2 lb density stuff makes a lighter weight sponson but will not last. I have also used Rohacell IG in a 1 lb density/cu ft. The Rohacell foam has a very high compressive strength.

JA