Your best bet is the "car body filler" type of bondo. It is not difficult to work with at first, more like the consistency of thick mud or the like, but once it dries, be prepared to sand sand sand sand sand sand sand sand sand sand sand sand sand....and sand some more.
I've never heard of people using spackling to be honest. Like you I've always heard bondo. Use bondo, apply and let dry in an open space, and either buy or borrow a rotary or mouse sander to help you take down that bondo to a nice smooth finish.
Remember heavier grain first and then move it up gradually to the finer grain. I've always heard 80grain to begin with and then move up gradually to 300 grain.
You will end up with some air bubbles and low spots, for that use the glazing putty they sell right next to the body filler stuff. Also, make sure you get another stick of the hardener. The one supplied never seems to be enough to finish the container.
I haven't personally used this stuff yet, I'm still in the build sintra framework part, but I've been gearing up for it. A good person to ask about this would be Mr. Jaiden. He has recently done it and has worked with the stuff before if I'm not mistaken. Look for his thread named "My attempt at a WOF scratchbuilt...". A lot of good pictures and good advice.
Happy Hunting!
~Bio