I would just select some vertices in edit mode and use proportional editing to move them down a bit to make a rough indent. You probably don't need it to be exactly form fitting. :)

You could also try using dynamic paint (just google and you'll find tons of video examples) to either create a displacement map or actual displacement, but I think the proportional editing route is probably better in this case.

Another option might be to bake an AO map and use that to create a displacement map - it won't help much with making a morph but might look good enough to use depending on your purpose.