Stormin'
Just use a razor saw, knife or other, and cut the old wood out. Be careful not to poke the tank.
As for foam: I'll get some flak for this, but I've said it before, If you pack it GENTLY you can use just about anything. We make a "crash video" and even though MOST of the crashes were genuine, we augmented the video with some "made to crash planes". In those planes, we just lightly wadded up news paper, or some we used bread bags, they worked fine.
ALTERNATIVE SOURCE: You can look in the yellow pages, and find an upholstery place, go to them, (or their dumpster) and pick foam that feels soft and uh, gooy, -like the fake barf or dog poo you find at the gag shops. This will be excellent foam, as opposed to the foam that feels dry, and coarse to the touch.

Jetts