The inside of the cover is made of the same material as old-fashioned ice pillows, and it's a sturdy construction, typical of Fujilatex. I tried putting a little air in it, and it doesn't seem to leak, but it's old, so it's being sold as-is. Please understand. Diameter: Approximately 40 cm. ※Note※ The box is missing, but the usage instructions are as shown in the second photo.