You got it right. Great stuff "Big gap" foam. Use painters tape and mask all around the end of the stock. Probably should tape a plastic bag or something around the stock as well. That stuff is messy and is tough to get off whatever it dries on.
It actually helps the curing process to spray a fine mist of water in the stock first, because it is a polyurethane base (similar to Gorilla glue).
Fill halfway or so, let it fully cure, and then cut off the excess that expanded out.
Whatever you use will add weight, but this shouldn't be much over an ounce at most.
