foam filling a 1990 TMNT mask for a perfect fit??

Hello. I'm getting ready to cast a tmnt mask in liquid latex. I have saw other ninja turtle mask that were latex skinned backed with 2 part expanding foam to create a perfect fit but I'm not aware of the process. I've spent hours researching but the only information I've been able to find is instructions on foam filling mask for display purposes. Any advice would be greatly appreciated.