If you can't find access, you are going to have to cut an access hole and retrieve the plate. As stated, you don't want it rattling around in there.
If I recall mine (almost the same thing happened to me) was a 1/4 inch plate with three holes drilled and tapped.
It will be easy to make using the old one as a pattern.
Then, if you are lucky the bottom clip will still be intact, you just fish the new one back in place.
If not get the plate in place and bolt it down with the three hinge bolts. Now drill a fourth hole (10/32 works) off to the side and tap it. Put a small bolt (short as possible) in the new hole. Use this small bolt to hold the plate in place until you get the three hinge bolts started. Then remove the small bolt and align your door. This way you will still have the movement necessary to make your adjustments to the door.
Just don't remove all the bolts at the same time or it will drop to the bottom again.

Last edited by Rusty64; 02/01/2011 2:04 AM.