I can relate my experience with RHtubs when I bought a replacement cover back around 2010. I also had a big tub so I went with less insulation on top 3" maybe instead of 4-5? Those big covers get heavy. With regard to that site, I can say the measurement instructions were very detailed and helped me get a good fitting cover. It arrived in about 4 weeks if I remember, which was right about what the site said. I only kept the tub another 2 years, so I can't state how long that cover lasted.