The Optech Hood Hat is a neoprene cover which slides on the end of a camera lens hood, to help protect it from dust, moisture and impact damage. It also means there's no need to remove the hood in order to replace to lens cap to keep the front element safely covered. It can also be used for telescopes, spotting scopes and binoculars.
n.b. sizes quoted are the lens or hood diameter (leeway is +/- 1/4"/0.64cm).