Thanks! It wasn't too hard, actually. Much better than I anticipated. First I glued a plastic card profile (taking off the same width from the kit, of course). Then I used polyester putty, having made some grooves on the plastic for a better grip. It sands very well with wet'n'dry.
The photos should tell the story better than words. Take a look at this Flickr album: DC-6: correcting the Heller kit