Rubberized trunk undercoating question
 

I recently purchased a very nice 69 911T. The body is incredibly rust-free except for a bit of superficial surface rust in front of the gas tank area where the undercoating allowed a bit of moisture underneath. I plan to clean the area and use some POR 15 or something similar. What is the "correct" rubberized coating to put on top ? I'd like it to match the remaining original rubberized stuff. Thanks in advance!

Wurth SKS, or sprayable seam sealer would be the proper replacement, but the equipment to use is a bit pricey, there are a couple of alternatives, but not the original materials.