If you're having problems with it, you might look into taking sheets of ABS and heat forming them to the mold. If the heat doesn't work well enough, you could do some expirementing with spraying a little acetone/MEK on it to soften it, but obviously don't soak it. This would help prevent the air bubble problem, which should relate directly to the density issue.
Something else I'm wondering about... what is the source of the ABS you're melting? The thicker area's I did didn't seem to flex any more than the rest of the panel, but then again, I didn't have any large flat area's either. It might end up just taking more thickness to get what you're looking for.