Designed specifically to fit tubing of 5/8 inch (16mm), the 6020 Flexi-Felt® model is made of transparent and very durable plastic. The 6020 model is made in North America with our natural wool blend felt. Its an environmentally friendly alternative to tennis balls and other synthetic products. Once the unit is installed properly its very difficult, almost impossible, for children to remove it Our sled base models offer unsurpassed floor protection with the largest surface contact area on the market. Our patent pending antiroll system is guaranteed to keep the Flexi-Felt® well aligned.Designed for 5/8" tubing (15mm). Transparent, durable plastic. The product is guaranteed for 5 years.