Here's what I would do (And what I have done)

Squirt a littly of that "Spray Foam Insulation" into the hole (And I do mean a LITTLE! That stuff swells up to about 5 or 6 times its original size) Let it dry and don't touch it when it's wet (The stuff is sticky as hell!) It dries sort of like styrofoam. Once dry, you can cut it to shape. THEN, if you so desire, you could lay somefiller and/or cloth over it.

Something else you could try would be to lay some Waxed Paper over it to conform it to shape as it dries. But if you do this, be careful not to get any on the outside of the model, cause this stuff can be a bear to get off.