Plumber's putty vs. Estes clay:
I got the Estes clay in the mail yesterday.
I'm more impressed with it than I thought I'd be, but compared to the plumber's putty:
The plumber's putty is more malleable, heavier and sticks better. Not a huge difference but, without further testing, I'd go with plumber's putty over Estes clay.
I haven't done any temperature testing, but as someone else stated, Estes clay softens dramatically in heat. More testing needs to be done with the plumber's putty, but I'm feeling pretty confident in using it as a substitute for Estes clay.
One test I might do, that requires ruining a nose cone, is to cut off the end of a nose cone, run a string through it, tie a knot on the end, so it can't pull back through, pack some putty into the nose cone (trying not to come in contact with string), and then swing it around my head, at high speed. This would create centrifugal force, trying to pull the putty out. If it still doesn't come out, then I'd feel confident using it on some fairly powerful rockets.
I might go out in a large open area for that test. I can see the nosecone coming off and breaking something.