I'm starting to think you're crazy, brother. You caught 30 waves on a board that has no major flaws and you want to fill in the concave? I just don't get it. But if you insist...
Typically you use pour foam to fill in foam, which you then shape and glass over. Since the strength is in the glass, and you're not pounding the bottom of the board with your feet, you only need 2 lb. foam. Higher density foam only adds weight you don't need.
Your project is not an easy or quick one. You should be pulling off the bottom glass to do this, then glassing, hotcoating, and sanding. You need some serious skills to do this, and you risk screwing up your board to the point of no return.
Or... you could go old school, and just use Bondo!
Once I get the property density of pour, I guess my biggest issue is how to keep this thing from blowing the lid off when I try to seal it in. I don't really know how well I can contain the pour inside the concave of the board. Obviously, I will need a clever way to either pour through a hole and seal it. It sounds like a challenge but I don't think it's real difficult - I am only flattening about 3 or 4 feet of the board - essentially, the front section (towards the nose) of the board.
Sounds like you are dying to shape a board. Get a blank and do it. Sell the other one before you ruin it. 2 sawhorses, a grinder and a tree for shade. Can't get anymore soulful than that.
I'm not really crazy - just patient since it's not an emergency. But catching waves is one thing, ..., the board stinks. I have other boards that are better. I just want this particular board to be better also.
But I might just make a concrete cast by using some spars for the shape and then pouring into that outside with a real tight lid. So if it tries to blow the lid, no big problem. The hardest part here is to get the right shape. I could first make a plaster or bondo cast then use THAT shape to make the concrete mold. Then pour into the mold. Tedious but at least there is 0% chance of ruining the surfboard since all I have to do is peel the foam off the concrete and set it in the concave with some stickum, then glass over it. Might need some wax paper or surfboard wax to make sure things peel off smoothly. But this way I don't have to surform off a heap of foam.
If I just pour some in the concave, it will rise pretty darn high and involve a lot of surforming.
gotta see this board
Post a pic. Seems like this is a longboard that uses the concave for noseriding.