Rear Deck Regal

what we usually do is build a new rear deck out of 1/2" MDF, If there is anyway that you can salvage the old one just enough to outline and get a template, that would be the easiest. If not, then use a piece of cardboard and trim to fit the rear deck, transfer to MDF, and you should get the results you are looking for.

try any local hardware or lumberyard. MDF is Medium Density Fiberboard.

Can't u just mount those speakers in the trunk?  Hold them into the opening of the rear deck, and screw up into the sheetmetal.

my buick, beit a lesabre, should be similar in mounting i would assume. and i mount from inside the trunk where the 2 stock baffles were. you didnt try and access the speakers from the deck itself in the cab did you? you may have to make the cutout chad mentioned so you have the rear deck. The mounts in the back arent connected to the cardboard, they are connected to the unibody and accessing them is easier than i've seen in most cars, since you can fit into the trunk itself and work on things hehe :-). hth