Featured for use as a part of a reaction canister of an inflatable restraint system is a conventionally extruded one-piece canister structure body part having first and second opposite side walls and a bridging portion joining the first and second opposite side walls. In the body part, at least one of the bridging portion, first side wall and second side wall includes a tear seam adapted to tear under the influence of a build-up of pressure thereagainst to provide venting of such pressure. As a result of alleviating concerns such as of material fragmentation when exposed to extreme heat conditions, e.g., a fire, the invention allows the more widespread use of light weight, temperature sensitive materials in reaction canister constructions.