A coolable double walled structure, for use and exposure in a hot gas flow environment, includes a jet issuing wall, a target wall spaced from the jet issuing wall defining at least one cavity therebetween. The jet issuing wall includes at least one impingement rail having a greater thickness than the jet issuing wall such that a bottom portion of the impingement rail extends within the cavity formed between the jet issuing wall and the target wall. A number of impingement ports are disposed within each impingement rail to provide flow communication through the jet issuing wall to the target wall. Due to the relatively thick dimensions of the impingement rail, the impingement ports disposed within respective impingement rails can be directionalized so as to provide for local cooling needs. In one embodiment, the entry areas of the impingement ports are bell shaped so as to lower inlet pressure loss. In another embodiment, the exit area immediately surrounding the impingement ports is rounded for lower post-impingement pressure drop.