I had this problem too. pulled up the carpet and my cat. was looking at me. I sanded (the whole inside, some surface rust) and used a rust prevent. from nappa. Then I used some scrap sheet metal and pop-riveted it in w/sealer. Went back to nappa and got some spray on liner, 6 cans to be safe, tape off trim and went to work. I used about 4 cans and is close to .25" thick. I got tired of driving around with no carpet so I cleaned the old one and put it back in. I can feel a huge differnce now, Quieter and the inside stays cooler. Also with the liner in it any standing water that gets trapped under the carpet wont cause it to rust again.
buster