You can also use Spray Foam and Felt. We do custom cases at work a lot and we will spray the foam in (start with very little and let it begin to expand) and then place the felt over the foam, press the pieces you wish to fit and allow it to form. You can use a razor to trim if needed once it has dried. It can get a bit messy but it allows you to get a really tight fit.
where do you get that stuff??