You had great success with an electric knife on closed cell polyethylene foam? The foam that doesn't compress much?
If that's the case, then I'm not that worried about it. Drafting up a rough design right now, just found a major problem with my AR, the handguard is too wide!!If I replace it with something else, then the AR will fit (tightly) in a 2" wide slot, then I could have 4 slots that are all 2" wide. The 4th slot could hold pistols with a 1" piece of foam to tighten the pistols in place.