I use the GS gap and crack filler and sometimes the BIG Gap and Crack filler.
The Tetra Waterfall Foam is also nice since its black but 3 times the price.

Keep in mind when putting the foam in that it expands 30 to 50 percent larger than what was sprayed in. If your going to sculpt it let it really dry for a fw days.
If you have places that explode with a large amount from underneath, let it go and just pull that off once it drys. If you mess with it you'll have a mess.