I cut a plywood cover for the opening and a smaller cardboard inset.
My assistant glued the inset into position on the plywood.
I used a drill to screw the plywood onto the rudder
and drilled pour holes.
The two-part foam components which become active when mixed 1:1
Pour together, stir throughly and start pouring within 45 seconds.
Here, I'm holding the pour spout waiting for my assistant to do the pouring. The foam grows to 8 times the original volume and gets rather warm.