A great deal of costume armor in the cosplay world is made out of foam, so I would start with Ted Haines's foam fabrication lessons on making a T-Rex. While the subject matter is not the same, the tools, material, and techniques apply to all foam fabrication.
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https://www.stanwinstonschool.com/tutorials/make-a-dinosaur-t-rex-foam-fabrication-part-one
/Chris