I switched to this stuff. It lasts a year or so if it's red; the black ones last much longer. A good feature is they don't fatigue when set up in a stretched condition. "The Better Bungee"
This is the “solid elastomer” stuff. I don’t think the McMaster shown above has it listed. I use it for return for constrictor clutches. Stuff lasted something like 3yr in sub 30 latitude sun.
I get the shock cord they use for spear guns, solid rubber, 8-12mm, small hole in the middle, good dive shops will have rolls of it to make up strops in custom lenghts. Doesn't last long in the sun though.