replacing speakers in a ford focus 2005 zx4 "Hurry plz"

Hey guys - I am replacing my rear speakers in the trunk and have no clue how to get that sheet of metal down. its like a big long wide sheet of metal and
it looks to be welded so I have no idea on how to get it down. I have been told when replacing the stock speakers you have to buy some kind of gasket or ring to find the after market speakers. Is this true?
Thanks - guys!

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