On the expanding foam trick, one tip is to fill the figure with water and
then empty out and give a good shake to leave tiny water droplets all over the inside,
seems this stuff goes berserk when it hits water and this ensures and even and
complete fill though make sure you leave some small holes for
air to escape and excess foam to leak out.

One my friends tried it without doing that and boy did that Jason figure
inflate and come apart in spectacular fashion.

Gave us a laugh anyways.