bondo was born automotive body filler. there are a couple of variants now, but i couldn't tell you the difference between them functionally.
you can get it at home depot in the adhesives area or any auto parts store.
as far as sculpting in it, that could be a challenge. it starts off as a paste, maybe a little more workable than cake frosting. when it starts to set up, it goes REALLY fast... from paste to rock faster than you can say "grrr! all this bondo wasted!!!"
how big a piece are you looking to sculpt? my favorite quickie sculpting medium is epoxy putty, available in the plumbing section of home depot. it's about $6 for a tube that's about 1/2" diameter and maybe 7" long. you cut a slice off, knead it together, then go. you get about 5 minutes where it's easily workable, then by 10 minutes it's solid as a rock.
for small stuff, it's awesome.
