I cannot attest to the effectiveness of PVC/carpet with regard to anti-warpage but I CAN with the cradles. I was MOST concerned when I found out I'd lost the indoor storage space where I kept my PA 14 for the last 2 years, especially as I watched temps climb into the upper 90's and beyond over a period of 6-8 weeks. I kept going out during the heat of the day to check for warpage around the contact points of the cradles and was so relieved to find it was a non-issue.
I know most companies recommend the use of their products/accessories to avoid warranty issues but in this case, and knowing how Hobie backs their products, I thought it wise to use the cradles and am MOST satisfied with their effectiveness. Are they pricey? YEP! But, I'm glad I sprung for them.
If I decide to buy another PA, I'll also be purchasing a set of cradles!