I got mine at EB Games. Not all EB Games got them either. Not yet anyway.
As for the coffin, there is a peg on the opposite side of where it is hinged. That peg fits into a peghole in Arnold's shoulder.
Mine fell once already today. The trick is, take the weapons out, and try to balance the coffin to where there is more weight on the back then the front. Then it looks perfect.
That damn Stealth Predator will be mine someday. I've got the other two, plus the Alien/Predator set in the mail, and I'd really like to round out the set.