Replace your old soggy headrests with new ones. These headrests are found on Spaform and Aquamarine hot tubs, as well as other brands.  They clip into place with two pins on the back, which insert into notches in the spa shell.

Grey colour.

Dimensions
The distance between the pins is 15cm (6").  The pin diameter is 10mm (3/8").  The length of the headrest is 28cm