I have an 84 300d that is in pretty great condition, but after taking it to the car wash today, I notice that the floormats on the passenger side were soaked.

I pulled up the carpet, and found out that the hole on the rear passenger side that is used to blown in that rust prevention stuff during manufacturing has lost the flexible sealant that normally covers it.

I am wondering if anyone knows of a good size plug or replacement material, so that I don't have water finding its way into my floor again.