There are all kinds of foam. Styrofoam which is a Dow  trade mark (blue foam color is a tm too) is widely used generically. The florists foams are lower density and strength. The pour foams I have heard of are poly urethane based and in 2# density shape easily but are not dependable for core, too brittle and when hot wired produce poison gas. Pour foams can vary greatly within a pour.  Billet foam with the same chemicals and higher density can work well. It takes R&D and testing to prove. Following plans should be safe, given decent workmanship. Changes become much more experimental.