I have a NordicTrack. The running board was causing a horrible noise. There is a sheet of plastic over the running board that has crack/tears. I could see all I needed was that plastic sheet. I couldn't see from any posts how to replace this part. I called NordicTrack and asked them and two told me I needed to replace the entire deck, $250 plus $60 shipping. I was told the new board comes with the plastic sheet. On the second call I asked if there was anyone to ask if I could just replace the plastic sheet or if they could tell me what material was used. She did and returned stating that they have a "Mylar Fix Kit." The kit was $50 plus about $15 for shipping. Sounded good to me. I ordered (happy I wasn't having to pay for the entire running board. I got off the phone and looked up the Treadmill Mylar fix kit. $17 + shipping. I cancelled the previous and am ecstatic I found the solution. If someone else put it together, there is a way to repair it.
Can you post a link to the Mylar Fix Kit you bought? Was it difficult to apply? Did it come with screws? I am replacing the running belt on my 4300R (20" belt) and noticed the plastic was torn. Contacting the company got me the same result as you