Sorry I didnt give a thorough reply i was rushing.
I have a tutorial on my site that is about Modifiers (1/43) on how to use Bondo.
Click Here To Read It its under Tweaks.
I just plop a whole bunch of bondo on the stock kit. Make sure you make a masking tape wall behind the bumper so the bondo doesnt slide off. Then just sand it into shape, or carve it with an X-acto knife or dremel.
I hope this helps now.