need to know how to seal trunk and where to buy replacement spare cover for 02 sentra GXE
 

I have a 2002 Nissan Sentra GXE and I was cleaning out my trunk and noticed everything was wet. I pulled up the carpet and also the (cardboard)? that goes on top of the spare donut, Everything was moldy. I removed the spare and there was standing water in the trunk. My questions are 1. What can I use to seal the trunk and 2. Is there anywhere that I can purchase another cardboard piece? Thank you