Well I did a whole rig with poly... http://www.infinity-customs.com/necronomicase.html
and that incudes the whole front bezel being sculpted out of poly, with one exception, the "face" at the bottom I first formed with wire mesh that I actually formed to my own face then worked the clay around and the top door is wood with a mesh insert (and no, it didn't bother it when I fired it, the wood didn't burn inside all that clay). The rest I just sculpted around the aluminum. The case itself is the same basic one the guy is talking about here, the only difference being the front bezel which I scrapped entirely anyway.
Actually there are meshes you can work with (metal) that won't give you the problems you've had. They're much thinner and more flexible and don't give you as hard a time cutting. Mostly they're used for clay work but they do work with bondo just fine, and it weighs next to nothing. A 12' run of the stuff barely weighs 10 oz. You can then cut it and bend it into whatever shape you like but you have to be more gentle when you apply that bondo because the mesh will bend easier, so use supports if you can for areas that'll be raised.
If weight isn't an issue, the clay is more useful for making detailed shapes and so forth. It'll add considerably more weight than bondo but the results will likely be more detailed. Plus you can work with it over a day, week, month, years, it never hardens whereas bondo will start to set up in under 20 minutes so ya gotta be quick.
Hmm, now, I wonder, do you think it's possible to do something to poly that'd let you powder coat it? Probably not, it'd have to be metal... But, now that I'm thinking on this track, ya know, they have stuff called PMC, Precious Metal Clay, it's real 24k gold and .925 (or higher I believe) silver. Expensive though, wouldn't try doing a case out of that.