This Engine Inlet Spike is designed for either Testor/Italeri's or Revell/Monogram's 1/72 scale Blackbird kit, and goes with my SR-71 Inlet (Testors/Italeri version, Revell/Monogram version).
I've designed the Engine Spike to be adjustable in length, as on the real article. Also note the air siphoning holes just past the Spike's maximum diameter. These would draw off thickening boundary layer at high mach numbers, drawing the air into the Spike body, and then expelling the air outboard, via the hollow cruciform struts connected the the bypass doors ahead of the engine face. Also provided are the air straighteners ahead of the turbine face, as well as the J58 turbine itself.
New to rapid prototype parts? Here are my general instructions for working with Fine Detail Plastic.