But, the 1,700°F is only for 5 minutes. My guess is that 0.035" steel sheet over 3/8" plywood would withstand that. Think about how flimsy 1-hour firewall (plaster) & doors (steel skins over wood frame) can be in buildings.
crthomps,
Don't re-invent the wheel. Try this site, they have Air Transport Association Spec. 300, Category 1 compliant cases, etc. Making the outer sheet SS rather than aluminum to handle the heat test.
Note the interior foam specifications:
Underwriters Laboratories, Inc.- UL Subject 95-HF-1.
General Electric Specification - SS268045D.
Lockheed Aircraft Specification - LAC-22-434A, Type II.
California Fire Retardency Urethane Foam Law - 1/1/84.
FAA (Federal Aviation Administration) Air Worthiness Standard.
MIL-SPEC. P-26514D, Type I, Class 1, Grade C.
There is also a Federal regulation: Motor Vehicle Safety Standard 302, aka MVSS-302. It requires a minimum flame resistance for materials used in the interior of a motor vehicle.
Try searching the Code of Federal Regulations: