I've used bondo with great success, just scour the area with a scotchbrite pad and some dishsoap beforehand to get the best bond to the plastic.
If you have a deep area to fill, put some cardboard into the recessed area. Bondo works best in coats under 1/4" thick, but I've pushed my luck and done well with 1/2". You can put one thinner coat on top of another and be fine, after it thoroughly dries. Sometimes it shrinks and cracks in thick applications.
If you need some reinforcement for strength, you can use the screen type fiberglass tape they use for drywall. That works great. Even window screen would work.
the bondo'd area should be primed before painting for the best job. It is a car body filler, but will soak up the first coat of paint and make the color look flat (second coat usually shines though). Finish sand it before painting, with some 600 grit paper because your sanding marks will show through the paint job if you don't.
I used it for what you intend, to "disappear" a second floppy drive bay, and to fill the fake vents around the front bezel. You can use your spot putty to fill any bubbles you find while sanding.
Hope that helps you out!
By the way, paper-mache isn't realy a bad idea, just a pain to get out the air voids you'll find while sanding it flat/flush.