Drag coefficient doesn't change that much. For example, a sphere-cone with a cone half angle from the axis of symmetry of 45 deg. the Cd = 1.1 at Mach = 5, For a cone half angle of 60 deg. the Cd = 1.5 at Mach = 4 . A half angle of 60 deg. is roughly the maximum allowable in air due to shock detachment. Millitary RVs typically have half angles of about 10 - 11 degrees. Hypersonic Cd does not have that much flexibility so the design parameters to play with are base area and mass. Ballistic coefficient, (beta) must be the same as the live RV for the decoy to work. Area scales as characteristic length squared, L^2. Mass scales as characteristic length cubed, L^3. If you didn't do anything special then beta would scale as L = L^3 / L^2 . To make a decoy work you have to break that scaling law. A nuclear weapon has a large mass (density) because of the plutonium pit. If you have the base area to play with then you can scale down the mass with a non-nuclear decoy (this is what they did with the decoys for the old Mk-6). A modern RV does not have that option.

This would make a nice lunch time conversation but this is a public forum so I'll end it here.

Gary