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Since I'm working on templates for my Jin-Roh costume. I noticed a lot of rounded objects that can not be obtained unless using bondo, or some sort of plaster. So if I wanted to round down an object to get it to look screen accurate, would bondo be the way to go? Here's and image of what I'm trying to get to with possibly using bondo.
I would cut out and shape as many pieces as possible to get the desired shape and then use the bondo to get the desired overall roundness.

But don't let the Bondo be your end all for a perfect smooth finish. Bondo can be pretty tough to sand especially when doing detail work.

Get the overall shape you want and then using their glazing putty too smooth it out. The stuff sands like a dream and there is no mixing involved.
Use spackle paste after building up layer to smooth things out. It will sand very easily. Then maybe a thin layer of bondo to seal it after you have the desired shape
Just tried using bondo, I am wondering if you build up the primary or base, porus or non-porous material? I just tried bondo on top of a plaster cast, and the first 5 minutes, the bondo cured weird. Like cheap elmer's glue after it dries on some smooth surface, it almost came off. I am just testing out primary or base materials. I guess I could try the spackle paste method. It's really messy when you sand it of course. Bondo, I thought was a filler like spackle paste, only you could get it smoothed even better. Is it better to pour it on somehow? It gets so gloppy. A 15 minute cure time has me working like a cake decorator for a Queen. I am used to sculpting with clay-like materials. So it's funny to see my unskilled bondo experience.

Edit ** After a full cureing, it became super solid on my plaster cast. That's cool. Wow, did it heat up too. I am trying to use cheap materials as a base do "diplace" the more expensive materials.
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I'm not sure a huge lump-o-bondo on your gut is going to be terribly comfortable as if you planned on just using Bondo to make that rounded shape, it's gonna weigh alot, a whole lot. My suggestion is a stab in the dark, but it seems to be the easiest and most cost effective route. Use foam and sculpt it to whatever shape you desire and then give it a finish coat of Bondo to smooth it all out. I think Bondo will dissolve some foam, so you'll have to try some of the white, green and my suggestion is try the blue foam that's most commonly used when affixed to concrete foundations for it's R-Value/insulation...Just a thought.

I think Bondo will dissolve some foam

I know for sure it will disolve foam picnic plates. Had a bunch extra and thought, "Hey...I could use these to mix Bondo on!"

It doesn't disolve them instantly or anything, but it will slowly eat a hole through white foam.
I know for sure it will disolve foam picnic plates. Had a bunch extra and thought, "Hey...I could use these to mix Bondo on!"

It doesn't disolve them instantly or anything, but it will slowly eat a hole through white foam.

Bondo will dissolve any kind of polystyrene foam---white expanded bead foam, blue or pink insulation foam, etc.

The solvent in Bondo is styrene monomer. That's a liquid made of single-unit styrene molecules or "monomers." That liquid will dissolve polystyrene molecules, which are "polymer" molecules made of chained-together styrene units.

You can seal polystyrene foam with epoxy so that you can put Bondo over it without dissolving the foam.
find something similar in shape, cover it with a couple of layers of masking tape, and fiberglass over it. Pull out the mold and you've got a hollow shape you can then apply a thin layer of filler on top. My brother does this all the time on car audio applications.
First, the catalyst for Polyester resin used in fiberglass and Bondo (MEK Peroxide) will dissolve many types of plastics and foams. Also Polyester resin does not chemically bond to plastics and doesn't like smooth surfaces. Mechanical adhesion is the way to go when using Bondo.

So, I vote for CombatBaby's method.

Or you can carve the rough shape from foam, put a layer of aluminum foil over it and then fiberglass that. Then just skim coat some Bondo to smooth it.
Why don't you make molds out of MDF and vacform the pieces you need ? you can also make molds and drape form expanded pvc (sintra) over them. This would give you the shape you want with no fillers and it won't weigh a lot either.
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