I don't have my source material immediately available, but I think I got the bulk of the information from some Navy drawings that Jim Ball sent me ten or twenty years ago. Over at Ye Olde Rocketry Forum, there is some discussion of Phoenix source material, and some scans of what is probably the same drawings that Jim Ball sent me many years ago. I don't know the exact profile of that curve--it is some form of secant ogive, where there is a corner at the base of the curved profile, but I don't know if that curved section is a circular arc, a parabola, a Sears-Haak profile, or some other mathematical optimization. I wouldn't put money on those official drawings having the exact profile, because that is not the exact purpose of those drawings. I pretty much eyeballed the curvature in my drawing, though I might have used some sort of measurement from a photo or drawing for the diameter of the cone at the mid-length point to try to get some accuracy. But to be honest, it's been at least four years, and probably more, since I did the basic artwork on that drawing.

There might be a tangent ogive for a larger body tube that you could saw off to approximate the shape. That depends on getting lucky about a nosecone with the right diameter and length, but there's a shot.