Get your hands on some pick-and-pluck foam sheet. I got mine online here. Just measure the inside volume of the case and order a little more than you will need.
Next figure out what you need to do to fit the foam snugly in the case. The thinner the layers, the more detailed the mold can be.
Put the camera on the foam and trace it with a sharpie. After a layer or two start making a void for the camera.
Add layers until the foam reaches the lip of the bottom half of the case. You can also remove sections for extra lenses and the carry strap. I just fold up the strap and sandwich it in behind the camera.
We'll assume you're ok with this, but you can opt-out if you wish.