I just rebuilt the rudder on my Etap 26 which also has a foam filled hull. I dug out the old foam and bought the 4 lb foam to replace it. That foam was very close in density to what came out of the old rudder. The foam in the old rudder was not water logged, despite a big gash in the rudder and it sat on a mooring for 25 years.
I found that you can't foam the rudder in one shot. I put a 3/4" polyethylene tube down into the rudder to about 1/3 from the bottom and poured the foam into that to get it to the bottom past the structural parts of the rudder. Then I pulled it out cleaned it off and put it in about 1/3 of the way from the top and did a second pour. I pulled it out and finished with a third pour.
Hope this helps.
Gary H. Lucas