Someone else may be able to comment on the proper foam, to use inside a rudder. I rebuilt mine while still on the boat, and wanted something I could pour into the voids, that would resist water, if any gets in.
I know you can get the mixable 2 part foam, and that may be the best route
, if you need to pour it.
What I did (time will tell how well it works) was to mix granual foam (closed cell, I believe) with epoxy, which bascially made a melted foam type material, that hardened once poured. Since it is completely saturated with epoxy, I figure it should be water resistant.
Anyway, I welded on about 4 extra stainless tangs to the rudder post and existing tangs / framework, then welded a bunch of gussets and cross-braces, so even if some water gets in, there is alot of welds that would need to be eaten away, before anything would give. (I used stainless welding rods, of course).
Then used epoxy and cloth / mat layers to put back together, and a few coats of clear epoxy as a water barrier. (Perhaps some other type of barrier coat would be superior).
Make sure you use different colors for barrier coats, bottom paint
, etc, so you can see how far you sand down in the future.