Is that stuff basically plumber's putty?
I don't actually put my bucket in the oven, heh... I sculpted my sculpey object around the earpiece then removed it from the earpiece to bake. Once cooled, I glued it back on and did all my tweaking with sanding and plumber's putty to fill gaps and make it all seamless.
Personally, I like using Sculpey for custom 3-dimensional objects. I have experience with it, it bakes solid, and it's very easy to sand. Also, the putty just dries too darned fast.
Plumber's putty is also a 2 part mixer that dries rock hard, but it dries in about five minutes, which is too fast for me to do any sculpting; that and it's sticky as hell, so it sticks to your sculpting tools.